I will suggest you try updating your device via i Tunes on computer.
Before go to i Tunes for i OS update, please delete the failed i OS software update on your i Phone/i Pad.
At first, i Tunes recognized the i Pod and the update appeared like it would complete successfully, but by the time i Tunes reached the "Waiting for i Pod" portion it could not complete the update.
An error message appeared claiming the i Pod could not be updated.
But my i Phone 5 had no problem at all going from i OS 7 to 8.