Q: update ios 8

Hi mohitmodi,

Thanks for visiting Apple Support Communities.

If you're not able to update your iPhone 5 to iOS 8 wirelessly, you may want to try updating using iTunes:

If you can’t update wirelessly, or if you want to update with iTunes, follow these steps:

1. Install the latest version of iTunes on your computer.
   
   
2. Plug in your device to your computer.
   
   
3. In iTunes, select your device.
   
   
4. In the Summary pane, click Check for Update.
   
   
5. Click Download and Update.
   
   

If you don't have enough free space to update using iTunes, you'll need to delete content manually from your device.

Find out what to do if you get other error messages while updating your device.

From:

Update the iOS software on your iPhone, iPad, and iPod touch - Apple Support

Best Regards,

Jeremy