How to update my iPhone 7 software?

Assuming you have a compatible device, 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. 

You can update an iPhone 7 to the latest release of iOS 15. You cannot update an iPhone 7 to iOS 16 or later, if that might be what you are asking about. For the iPhone 7, which originally ran iOS 10, iOS 15 is the end of the line.