How do I open my desktop tower?

[mattpie1]

I got an HP Envy Desktop - 750 - 140z CTO back in 2015/2016. In that time, while I have done my best to keep it as clean as best I can, I have noticed a major dust build-up in the interior of the tower. I tried blowing it out gently with canned air, and I took out one of the removable panels in the back to try to get to it, but no luck.

I think my only course of option is to actually open up the case, but I've been having issues. I tried unscrewing some of the more obvious screws, but quickly realized that those held the power supply, so I put those back in. I noticed what looked to be torx screws all along the edges of the back (small screw-like things with 6 points in the center hole), but none of my  torx screwdrivers will fit or they keep slipping. Finally, I noticed a single black nob looking thing on the lower left of the back, but that wont turn or unscrew. So now I'm utterly confused as to what to do. 

My question is what do I need to do in order to open up the case, and what tools I specifically need to do so. Any help and advice is welcome.

(P.S. - the warranty went out of date in 2017 if that's any help)

[GamingOptional · Accepted solution]

It should have a thumbscrew that looks a lot like that black knob, just grip it and turn it left(counterclockwise) it will loosen though it won't come all the way out, then the side panel it attaches to should slide out.