Why my iOS not updating.

See this support guide:--> Get help with over-the-air iOS updates - Apple Support

If, following all the steps provided in the above guide (included section "Remove and download the iOS update again"), the issue still persists, you might need to update that device using iTunes on a computer.

If you don't have a computer, borrow one from your friends, or ask for help at an Apple Store or Authorized Service Provider.

Regards

Giulio