Is replacement of back of iPhone done in store?

Apple won't replace the back of iPhone 7 or for that matter any iPhone. The back of iPhone 7 is made of a single block of aluminum, to which all the internal components are attached. If the back of your iPhone 7 is damaged, you can either purchase an Out of Warranty replacement phone for $319. Or given that iPhone 7 hasn't been sold in some time and was actually released in 2016, 5 years ago now, you might want to invest that money in a newer, more modern iPhone.

I don't think so. Click on Get Support on the upper right corner and discuss it with Apple Support.