Hi! Restart your device and try again.
Try to reset your device. This won't delete your data stored on this device.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
Alternatively, you may go to Settings - General - Reset - Reset All Settings
https://support.apple.com/en-us/HT201559
After that, try updating your device once again.