Upgrade Apple Watch
I just recently bought my Apple Watch series 3 in November. I did not buy the one with cellular. I have decided I would like the watch with cellular, is there anyway to upgrade my Apple Watch?
You can Trade In the Watch to Apple and get a percentage of its current value as a gift card you can use towards the purchase of a new watch.
However, you'll likely get more for it, if you sell it privately.
Upgrade Apple Watch