How can I install iOS 7 on my I phone 3GS - Cell Phones

http://www.imore.com/how-to-install-upgrade-ios-iphone-ipad-itunes

iOS 7 is not supported on any iPhone model prior to the iPhone 4.
6.1.6 is the latest version of iOS that is compatible with the 3GS.

The iPhone 3GS is partially supported by iOS 6. It is not supported by iOS 7 or subsequent versions of the operating system at all. iPhone 3GS 8 16 32 GB Specs iPhone 3GS A1303 MB715LL 2315 iPhone2 1...
Download iOS 6 1 6 for iPhone 3GS and iPod touch 4G

latest iOS possible:
iPhone 3GS = iOS 6.1.6
to update to iOS 5 is only possible with connecting the device to iTunes on a computer.
- Plug the iPhone to your computer and open up iTunes under devices click on your iPhone and then click on check for update.

If your problem concerns your iPhone 3GS (your cellphone), then no, you cannot upgrade it to IOS 7. IOS7 requires an iPhone 4 or newer (4, 4S, 5, 5C, 5S).

If your problem is about your iPad (your tablet), then you can't upgrade if it's an iPad 1. IOS 7 is compatible with the iPad Air, 4, 3, 2, mini and mini with retina display but NOT with the iPad 1.

There are two ways to upgrade the iOS version of your iPhone 3GS:
- via connecting the device to iTunes on a computer
- or directly from within the device, but only since iOS 5, it is possible to upgrade the iOS version without the need to connect the iOS device to a computer (over-the-air update)
just go to: settings - general - software update

The iPhone 3GS does support iOS 5 and even the latest iOS 6. However iOS 5 is no longer available for it by Apple's choosing, but the good news is that iOS 6 works well on the 3GS as tested