T-Mobile

Ok so me and my boyfriend got our phones from T-Mobile in February 2016. I got a iPhone 6s and he got a galaxy s6 plus. We are paying it off along with our phone bill we pay $160 sometimes less sometimes more. It will be helpful for someone to help me. I want too know can we trade phones meaning like give them our phone and trade for another one? Or we can't since we paying them off?