The Function key models do have a removable SSD! But Sadly, Apple uses a custom part and Apple is the only source for the part.
You'll need to go to an Apple Store to see if they will replace it under warranty if is bad. They will only replace it with the same sized unit. They don't offer upgrades!
As for upgrading you'll have to wait until one of the 3rd parties reverse engineer the SSD and offer it.
OWC was the first company to offer the replacement SSD for the other models they may do it if there's demand. But, this will take a few years.
Your other option is to buy a replacement system with the larger SSD and sell yours (or swap out the SSD's).