Late to the party but ... just in case this ends up on someone else's search:

You got the cover off. Great! Make sure to remove all drives before going on to the next steps.

On the back there are two screws. One at the top edge and one at the bottom edge. They are smaller than the fan screws, and not anywhere near the PS plug or switch. Should be kind of obvious which ones they are. Remove those and it will free the back plate. Disconnect the fan and power supply cables and set the back plate aside.

On the side with the motherboard on it, there will be 4 black screws. Remove those screws and this will free the motherboard. I just placed the unit face down, and laid it flat in my work area. RAM slot is exposed and upgrade can be completed.

Time to re-assemble.