Hi, I was trying to update my iPhone by installing iOS 11, but iTunes says error 3503, what should I do?

Yes, you can download iOS 11 Final version from Team Cydia.

Download iOS 11 Final IPSW for iPhone and iPad

iTunes error 9006 may appear when attempting to download, restore, or update an iPhone or iPad. Typically you'll see an iTunes error message that states something like "There was a problem downloading software for the iPhone. An unknown error occurred (9006)." Try the following tips to resolve error 9006 in iTunes if you encounter the error problem when updating iOS system software.

1. Insure that you have an active network connection and are connected to the internet
2. Quit out of iTunes
3. Update iTunes and also install any available software updates to Mac OS (or Windows, if applicable)
4. Reboot the computer (Mac or PC)

If above methods fail to help you out on the error 9006, get more solutions here: fix iTunes Error 9006 with 6 useful ways. Hope it be helpful for you.

There are several methods to update iPhone and following below steps can help you to reach this aim easier.

How to Update iPhone Software Wirelessly If a message says that an update is available, you could then click "Install Now" button to start to update your iPhone iOS software. Also, you could manually check whether there is an iPhone iOS update or not on your iPhone. The below steps would show you how to update iPhone software wirelessly:

1. Before you update iPhone software via WiFi connection, you need to make sure your iPhone has enough storage and battery to support the iPhone update.
2. Connect your iPhone to a WiFi network.
3. Tap "Settings > General > Software Update" successively.
4. Tap "Download and Install" option when your iPhone has detects an update available. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel. Later, iOS will reinstall apps that it removed.
5. Then iPhone would start to download and install iOS update immediately. You only need to wait for the update complete.

How to Update iPhone Software with iTunes If you can't update iPhone iOS software wirelessly throug WiFi connection, you could give iTunes a try. As the all-inclusive iOS device manager center, iTunes provides multiple effective assistances for you, including: backing up iOS devices, restoring iOS devices, transferring iOS data files, and updating iOS devices. You could follow the below steps to learn how to update iPhone software with iTunes in detail:

1. Install the latest version of iTunes on your computer.
2. Connect your iPhone to your computer via the USB cable.
3. Open iTunes and select your device.
4. Click "Summary > Check for Update".
5. Once iTunes detects the available update, click "Download and Update".
6. If asked, enter your passcode. Then iTunes would start to update iPhone iOS software. When it's finished, disconnect your iPhone with computer.

For more methods, check: How to Update iPhone

Yes, you can. The easiest way is to update the iOS system in the Settings app. Also you can resort to iTunes to do that. Follow the instruction below.

If you can't update iPhone iOS software wirelessly throug WiFi connection, you could give iTunes a try. As the all-inclusive iOS device manager center, iTunes provides multiple effective assistances for you, including: backing up iOS devices, restoring iOS devices, transferring iOS data files, and updating iOS devices. You could follow the below steps to learn how to update iPhone software with iTunes in detail:

1. Install the latest version of iTunes on your computer.
2. Connect your iPhone to your computer via the USB cable.
3. Open iTunes and select your device.
4. Click "Summary > Check for Update".
5. Once iTunes detects the available update, click "Download and Update".
6. If asked, enter your passcode. Then iTunes would start to update iPhone iOS software. When it's finished, disconnect your iPhone with computer.

For more methods, check: How to Update iPhone

Probably something went wrong, and your iPhone itself sees that too. That's why your iPhone asks to connect it to your PC or Mac with iTunes installed.
-Start iTunes on your PC or Mac
-Connect your iPhone to the PC or Mac
-Probably iTunes will advise to recover your iPhone.
>>If you didn't make regular backups of your data you probably will lose the data that's on your iPhone.<<
-Wait until the recovery procedure is done and see if your iPhone works again.

What if iTunes doesn't recognize your iPhone anymore in iTunes?
Thy a "soft reset' by pressing the ON/OFF button on top of your iPhone and the round menu button underneath your screen simultaneously until your iPhone reboots. Check if your iPhone connects to iTunes and try the recovery.
I hope this will help you solving your problem.

I think you mean OS 7.0.3. If you are downloading from the phone, try plugging in to the computer and updating through iTunes. Apple's update servers also sometimes get overloaded.

You have a situation where your phone needs to be recovered by plugging it into the computer where you sync it to iTunes. When you do this a prompt will come up on your computer that you have connected to an unidentified iPhone. It will ask you if you want to restore it from your last backup, or initialize it as a new iPhone. I recommend that you restore to the last backup, so you don't have to go to the iTunes and App Store to reinstall all your apps. The bad news is that if you have never backed up to a computer, your journey to recover your digital life is going to be difficult.

There are many ways to update iPhone. The easiest way is to update via air. Certainly, iTunes is another great choice. Just follow below steps.

Update iPhone to iOS 12 via Air
1. If a pop-up panel shows you that iPhone latest version iOS 12 is available, you can click on the "Install Now" button to start to update iPhone to iOS 12.
2. It is also accessible to check out if iOS 12 is available manually now. The following steps will instruct you to make it.
a) Connect iPhone to a good Wi-Fi network and make sure your iPhone storage is enough. You also need to ensure that the battery of your iPhone can support the update.
b) Click on "Settings > General > Software Update> Download and Install" successively and iPhone will start to download and install iOS 12 right away. Wait until the update is completed.

Update iPhone to iOS 12 via iTunes
1. In order to use iTunes to update iPhone to iOS 12, you can first download iOS 12 IPSW file from this website (click here).
2. Run the latest version of iTunes on computer and insert iPhone to computer.
3. Click on iPhone icon on iTunes interface and then hit "Summary> Check for Update".

4. Choose the IPSW firmware file that you have downloaded beforehand by performing the following actions in iTunes: >
Mac: OPTION + Click on the "Update" button in iTunes
Windows: SHIFT + Click on the 'Update' button in iTunes

5. Navigate to your computer folder to choose the IPSW file and then click on "Update" button to confirm that you would like to install iOS 12 in the pop-up panel.

6. Wait until the update is over and iOS 12 is installed on iPhone. In the end, your iPhone will reboot with iOS 12 successfully installed.

For more details, check: How to Update iPhone to Latest iOS 12