Iphone issuess?!?!

Gabriela • First time mom. 23. Due 06.26.17
I am going to upgrade/trade in my iphone 6s plus for the iphone 7 plus. I have to return my iphone 6s plus but it has two small cracks on the screen. Should I return it to them and have them charge me the amount or should I fix it before returning. I want to know what I should do? I just want to do whatever is cheaper.
Thank u