It's really just easier to put in a new one, but to disassemble, on the black button on the side there are 2 indentations which you need to use with a pin wrench. You might find a tool for this at your dive shop, but another hack is to take 2 allen keys that are big enough to fill the indentation but small enough to fit, and similar in size, and grasp them with a large adjustable spanner, and turn it that way.
Turns left to unscrew. You might want to take off the black button before or after, doesn't matter much, but use a large pliers for that. I think if you take it apart, you can hold the base of the unit with a small nut fitting and use that as leverage, but it has been a while since I've taken one apart. I just swap them for the most part.
The orings are an odd size, but don't worry too much about fancy orings, if you have something that fits from some harbor freight oring kit, that'll do. If the orings look ok but just had salt or dirt on them, clean and lube with silicone grease and put it back together. No need to remove the inflator fitting, there's nothing behind that. If it's not leaking air out, then no need to remove the other button.
