Upgrade iPhone 5 from iOS 7 to iOS 8
Yes this can be done you can download iOS firmware from http://www.felixbruns.de/iPod/firmware/

Once it is downloaded keep it on your desktop. Now put your device in DFU mode. (You can learn it from youtube if you dont know how to put your device in DFU mode.) iTunes will popup telling you it has discovered a device in recovery mode. Now press shift on your keyboard and click on restore. iTunes will start restoring your device. 

NOTE: you might get few iTunes errors during the restore.