i need to update my phone

The iPhone 7 can run the latest release of iOS 15 – but nothing higher.

A few days ago, Apple released a security patch to iOS 15. If your iPhone 7 is not already running iOS 15.8, you may want to update to it.

About iOS 15 Updates - Apple Support

Need to update it to what?

The iPhone 7 can not run anything later than iOS 15.