The Steam Deck's SSD can technically be replaced and upgraded, but Valve has recommended that users don't do this.
Yesterday on July 18, PC Gamer noted that a Reddit user reached out to Valve's Gabe Newell to enquire about the Steam Deck's SSD. Newell apparently replied to the user, stating that the SSD was connected to the Steam Deck itself via a 2230 M.2 slot, meaning the SSD itself can theoretically be taken out and replaced.
Due to the SSD's connection to the Steam Deck through the 2230 M.2 slot, you'd likely have to disassemble the entire device in order to replace the SSD. Through this, the user would obviously be voiding any warranty period on the Steam Deck, which is probably why Valve recommends you don't try this at home.
However, Valve has said they're working to improve the Steam Deck's operating system before it launches later this year. The manufacturer has specifically mentioned wanting to improve the operating system for developers, so there's always a chance this kink could be ironed out prior to the hardware launching (which we're really hoping is the case).
The Steam Deck launches later this year in December 2021, as Valve attempts capture some of that Nintendo magic that's worked so well for the Switch. Pre-orders for the new device are open right now, starting at $349.99, but scalpers are already auctioning off pre-orders for the hardware at ridiculous prices. You'll have to keep a keen eye out if you're still to reserve your unit.
