http://my.t-mobile.com should have the status of how much you owe.
Depending on how much you owe and how much credit you have available on your EIP, you may not have to finish paying off your current device before you get a new (you still have to pay it off, you can just pay for two at a time, possibly).
You can go to a T-Mobile store or upgrade online.
You also have the option of purchasing a phone from a third party (e.g. if you want to purchase a Pixel from Google or a iPhone from Apple, etc -- as long as it's compatible with T-Mobile's network and unlocked). You do not need to go through T-Mobile for this. I would recommend staying away from ebay / craigslist phones because they tend to get blocked (not all, but there's a lot of people who have come here saying "my phone doesn't work anymore" only to find out they bought a device and the original owner stopped paying on it).