Question about Joule Performance High End Gaming PC

I bought this PC and am very satisfied with its performance, quality and volume. How easy is it to expand this JP-PC with additional hard disc storage? Better M.2 or 2.5"? 3.5" is probably out of the question. I can't find any information on this, but I have seen in a YouTube video on a similar model that there are three 2.5" slots behind the right flap.