If you’re looking to do a direct trade-in, contact the Tesla company or dealership. Basically, you’ll give it back to them, and they’ll put that value toward your next purchase. The only thing to watch out for is that Tesla has gained a bit of a reputation for undervaluing their vehicles when taking them back.
Your other option is to go to a non-Tesla company buyer, where you could get a really good value, but the process could be a little more time-consuming.