Turn off the machine, close the lid, and flip it over so it's laying upside down, hinge away from you.

Remove the ten (10) torx (T5) screws around the edges of the case.

Flip up the magnetic "lid" that says "Precision 5530" to expose the service tag and two Phillips (PH1) screws. Remove these.

Starting from the hinge side, slowly lift the rear case off. There are a few snaps, this is normal. There are no cables attached, so when it comes free, put it off to the side.