Can you trade in a iphone 5s towards an iPhone 6 plus at tmobile?

My iPhone 5s is an really good condition

If it's a financed phone, you'll have to pay out the remainder of your contract first. They couldn't care less what you do with your 5s.

If you own it outright, then they may give you a trade-in value if you agree to renew your contract.

Keep in mind that the prices of new 5s phones dropped. This will directly affect the offered price for yours because no one wants them (just like you).