Software update

Go to Settings > General > About. Are you running iOS version 14.7?

If so, your running a beta version and will need to uninstall it first.

How to uninstall iOS or iPadOS beta software - Apple Support

If you're not running a beta version, there may be other issues, 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).

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Are not using a VPN connection.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 6 gigs of storage available.

You can also check out this article for help -

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad or iPod touch won't update – Apple Support (UK)

someone else in these forums had a similar issue this morning.. and it was because the display was zoomed in and was unable to scroll down to see the download/install button. So you could try turning off zoom or you could update using a computer