Apple Watch can I upgrade for cellular?

How long ago did you purchase it? Did you purchase it directly from Apple, or another retailer? Apple's return policy is 14 days for a full refund. After that, you would have to sell the device on your own and then use the money to purchase a new Watch.

How long ago did you get it? You would need to receipt from its purchase, and to know where it was purchased. To be honest, getting the cellular version is a waste of money. The Watch costs more and if you activate cellular, you have to pay an additional monthly fee to your carrier. In the US that is $10/month. The GPS only watch can do everything the cellular one can do as long as the paired iPhone is nearby. Do you really see yourself not having your iPhone nearby that much to justify those costs?

iPhone nearby, is generally somewhere in the same home, where home is typical wood frame construction.

Also if both iPhone and Apple Watch are on the same WiFi network, they can keep in touch that way.

My Wife’s iPhone is generally in the kitchen, and her Apple Watch gets texts, notifications, current weather, phone calls, etc... when she is in here office.

As long as your iPhone is nearby, the GPS only model will do everything the cellular will do

I understand that Pandora has been updated to allow streaming on the cellular iPhone, but it and Apple Music are the only two. That is one case where you would need the cellular version if you don't wish to take the phone on the run. But then, what have you been doing in the past? In the house should not be a problem, so long as your iPhone is on. Make sure that your iPhone is a model that will support your Apple Watch. Most Watches now require an iPhone 6s or newer with iOS 13 on them.

Streaming needs a network connection. Either cellular, WiFi, Bluetooth. I assume your runnig route is not covered by public WiFi.

Pandora, also has the ability for paying customers to download offline playlists.

And of course your own music, plus podcasts and audiobooks.

Then you might want to look at downloading music to the Watch from your iTunes library, or as Bob said, Pandora if you are a paying customer, and you can pair your AirPods to the Watch and listed as well.

I do.