Recently I took the initiative to upgrade my steam deck hard drive from the 256gb it came with to the much larger Micron 2Tb 2400 that was reasonably priced at Amazon (not an affiliate link).
There’s no shortage of videos on YouTube to help you out, but I often find written guides to be more passable.
Daniel over at WhoIsThisJoker was kind enough to write a guide for the process using the pictures I provided him.
I found this guide very helpful and I think you might also if you need to upgrade you SSD.
The only thing I might add is that when unscrewing all the tiny screws around the outside of the deck it works best to unscrew them all about 1 full turn at a time and move on. This prevents working your way down to less and less screws maintaining pressure against the back plate and reduces the possibility of stripping the screws.
Happy Modding!