Unable to Update IOS 15.7 in my 7 Plus

Try updating using a computer.

Update using your computer -

Update iOS on iPhone - Apple Support

What version of iOS are you currently running?

Go to Settings > General > About and look the version.

You may have a beta version installed. If so -

​ How to uninstall iOS or iPadOS beta software - Apple Support

You need to have a WiFi connection to install it wirelessly on your phone.