I have an Applewatch Series 10. How can i trade it in?

You have 14 days from the original date of purchase to return it for a refund.

After that, it's yours.

If you choose to sell it, please follow the instructions here: What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

That's because it's not eligible for trade in at this time. The 10 is the current model. You can't trade in the current model.