It’s not hard at all, if you’ve done any kind of tech disassembly before you should be good. Just make sure you don’t strip the screws (don’t use a screwdriver that’s too small, make sure it’s all the way in the screws before turning it).

You will have to either clone the drive or install SteamOS fresh on the new SSD from a USB drive. They may have fixed it, but originally the SteamOS installation/recovery USB had a software bug that would crash the wifi driver if you connect to a 5GHz wifi 6 network. So if you have a WiFi 6 network, I would suggest only connecting to the 2.4Ghz version of it until you’ve completed setup and downloaded updates.