It's very easy if you've done anything like this before. I would definitely follow a full guide, but the basic process is that you have to remove the microSD, take a few tiny screws out, pop the back open, unplug the battery, take a few more screws out, remove the heat shield, swap the SSD, and then put it back together.

After that you need to reinstall steamOS on the new drive, so you need a way to plug a USB drive into the deck (a dock, type-c adapter, or a type c USB drive). You can install the steamOS recovery software to it following the online instructions, boot the deck into bios with the USB drive plugged in, pick to boot off the USB drive, and then hit reinstall steamOS once the recovery desktop loads up.

Only part where I've seen people have problems is by stripping out the screws on the back of the deck. You have to make sure you are using an appropriately sized screw driver and that you have it fully inserted into the screw before turning. This usually isn't hard to do, but if you strip a screw it can be hard to remove it.