You've already removed a black screw in the back left corner at the top of the printer, four push clips along that left edge that secured the back panel to the left panel, and three push clips along the top edge that secured the top panel.

From the top of the printer, at the back right corner, remove the black phillips head (+) screw. This is one of the two longer ones. There are two others in front of it, but those aren't attached to the case, so you can leave them in.

Inside the printer, there are two black screws attaching the back panel to the floor plate, each with a small washer. THESE ARE HEX HEADS, not phillips. You can loosen the left one without removing it, but I found that I had take out the right one entirely to get the back panel off. Screw it back in loosely for safe storage.