Valve has disassembled the Steam Deck in a teardown video. However, the company hopes that the video will serve as a deterent, rather than as a teardown guide. Nonetheless, replacing the machine's M.2 2230 looks relatively simple.
https://www.notebookcheck.net/Steam-Deck-teardown-reveals-the-modularity-of-Valve-s-handheld-console.570105.0.html