My iPhone 7 battery life is 79%

That part is easy. There's one in every Apple store, and in the US Best Buy is authorized to replace batteries.

Genius Bar Reservation and Apple Support Options - Apple

To find a store: https://www.apple.com/retail/

If you mean Battery Health is 79%, then yes, you should have Apple replace it. Make a genius bar appointment; they can usually do it in about 30 minutes, for $49.

That's definitely the time when Apple would recommend it. Way back when, Apple or an AASP wouldn't even consider replaceing a battery unless the battery health was reading at less than 80%.