How do I replace my battery in my iPhone 7 Plus?

Make an appointment at an Apple store or AASP, bring it in and pay them to do it. Apple doesn’t sell parts to consumers.

Make a Genius Bar Appointment:

Genius Bar Reservation and Apple Support Options - Apple

If the battery is bad there should only be a $29 charge for replacement.

You make an appointment at an Apple Store or authorized service center. You cannot do it yourself.