Apple Watch

Go to the Apple store app on your iPhone and follow the prompts to purchase a new watch.

It should give you the option to trade in your old watch and will give you a dollar amount.

You can choose to proceed or to cancel.

In addition to what Randy-12 correctly suggests, you must know your current Apple Watch Serial Number in order to get the trade value. You can find that:

1. On your watch, Settings > General > About
2. On the Apple Watch app on your iPhone - General > About