How to update my iPhone 7?

iPhone 7 can be updated to iOS 15.8.1 but not higher.

See how to update your iPhone here --> Update iOS on iPhone - Apple Support

If you have skipped a lot of updates, you may have better success updating using the directions provide about updating using your computer.

If you want higher than 15.8.1, that is not possible.

Please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Here are the instructions:

Update your iPhone or iPadApple SupportOfficial Apple Support › en-us