How to update iPhone 7 to latest iOS version?

You can update your iPhone to iOS 15, go to: Settings -> General -> Software Update.

See this Apple article: If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-us/HT201435

Or update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

https://support.apple.com/en-gb/guide/itunes/itns3235/windows

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

https://support.apple.com/en-us/HT212185