Can I exchange my cracked iPhone 6 and get an iPhone 6s?

I have an iPhone 6 currently connected on T-Mobile and I dropped it a while ago, it fell and got seriously cracked on the screen. Not so bad that I can't use it but still looks bad. Would I be able to go to t-mobile or apple and have them take my iPhone 6 in exchange for an iPhone 6s? With extra cost of course.

You should get the screen fixed first. You will get more money for the trade in value of your phone. You may even get more money selling the phone to someone else then using that money towards the purchase of a new phone.

Ask T-Mobile or Apple.

You will probably have to pay a lot extra, maybe get a 2year contract

Yes but the trade in value of your phone now goes way down.