What happens when you try?
Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.
Make sure you:
• Are using a WiFi connection (not a hotspot).
• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).
• Are not using a VPN connection.
• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.
• Do not have power save mode turned on.
• Do not have a beta version installed.
• You have at least 10 gigs of storage available.
Click on this link on what to do if you can't update your phone -
If your iPhone or iPad won't update – Apple Support (UK)