Cant update to ios 8.2 - Apple iPhone 5

For updating the iOS, you need to be connected on the Wi-Fi.

Tap Settings > General > Software Update. Tap Download and Install. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel.

1. Over the air:
1) On the homepage of your iPhone iPad or iPod touch, open Settings and click the "General" option, and then select "Software Update".
2) Click the "Download and Install" button to begin download the iOS 8 installation package.
3) After the iOS 8 installation package is downloaded successfully, click "Install Now". Then your device will restart, and you can experience the latest iOS 8 immediately!

2. With iTunes:
1) Connect your device to your computer, and iTunes should launch automatically. If not, open it on your computer.
2) Click your device icon from the left side bar, then tap the "Summary" tab.
3) If there is no notification about iOS 8, click on the "Check for Update" , and then click "Update". When the download is complete, your device will restart and install the new OS.
How to Install iOS 8 on iPhone iPad iPod touch iMobie Guide

try here
Update the iOS software on your iPhone iPad and iPod touch

Did you jailbraked your iPhone with iOS 7 on it? if that's what you did then it won't really update the phone because it has a modified iOS. If you really want to update it to iOS 8 you need to restore your iphone hence you can now update it to iOS 8

Have a look here:
http://www.macworld.co.uk/how-to/iosapps/how-downgrade-from-ios-8-ios-7-reinstall-ios-8-3522302/

must be because of an old iOS on the iPad. some games require iOS 6 or 7 or even 8.
- if you have an old first generation iPad (without camera) you can only update to iOS 5!
- an update depends on the iPad model you have:
For the iPad first generation (A1219) to update is only possible with connecting the device to iTunes on a computer.
- but this model supports only iOS 5
- NO iOS 6/7/8 !
-----------------------------
for all other models:
There are two ways to upgrade the iOS version of your iPhone, iPad, or iPod Touch - 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

on a 1.gen. iPad it is impossible to install iOS 6!
this model supports only iOS 5 !
- NO iOS 6/7/8
- the processor is too slow....
For the iPad first generation (A1219) before iOS 5 to update is only possible with connecting the device to iTunes on a computer. Plug it into your computer and open up itunes under devices click on your ipad and then click on check for update.
- but this model supports only iOS 5
- NO iOS 6/7/8 !

For the iPad first generation (A1219) to update is only possible with connecting the device to iTunes on a computer.
- this model supports only iOS 5 - NO iOS 6/7/8

First Generation iPads can use iOS 5
- but iOS 6 / 7 / 8 need an iPad 2 or newer and are not compatibel with first gen. iPhone
-updates can be made in iTunes connected to a computer or under settings - general - software update
Upgrade iOS from within iOS device
(valid for iOS 5 and above)
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). To upgrade to iOS 6/7/8, simply navigate to Settings > General > Software Update. The screenshot below demonstrates how to do so on an iPad, but this is also valid for the iPhone and iPod Touch.

in iTunes:
How to update to iOS 7

plug it in to itunes and when u pull up itunes it should say an update is available for your device or something like that, then update it. it will update it to iOS 4.2.1 so you will be able to get most of the apps.