I want to update my Apple 7

An iPhone 7 cannot be updated past iOS 15.7.9.

If a sensor isn't working, that is most likely a hardware failure. Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple

Twumasiclinton Said:

"I want to update my Apple 7: It is reading slow and the sensor is not functiona"

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Updating an iPhone 7 Past iOS15:

A. You can't.  The iPhone 7 will only install to iOS 15.7.9. See here: Update your iPhone or iPad Software if you're Experiencing Issues with Location, Date, and Time - Apple Support. And, for the note, even if you could update, only the latest compatible version will install.

So, what to Try:

B. Update Using a Computer:

Have a Computer Handy? If so, try using a computer to update. Use my User Tip Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough