Change apple care

Are you sure what you're paying T-Mobile for is actually Apple Care, and not some warranty add on by T-Mobile?

If you enter you device's serial number in the following link, does it show that you have Apple Care+ on it? link-> Check Your Service and Support Coverage - Apple Support

If it is Apple Care+, then you would need to contact T-Mobile to see about cancelling it with them. You only be able to add it back on, if its been less than 60 days since you bought the device. If its been more than 60 days, then your device would no longer be eligible for Apple Care.

Enter the watch's serial number at the website linked to above to verify. If it comes back with Apple Care+ then it already has it and you would need to cancel it before you can get it again.

See here for how to cancel it: link-> Cancel or change the payment method for your AppleCare plan - Apple Support

If it does not show up at the Check Service and Support Coverage link provided above, then you will need to contact T-Mobile to cancel it, but again, you may not be able to get it again on the watch if its been more than 60 days since you bought the watch.