Peel the rubber feet off the back to expose the screws. Undo the four screws, and lift the back case off of the front. Then lift off the bezel, being careful to note its orientation. I suspect this problem must be common to lots of HP-41 series calculators. The screw posts are under a great deal of stress, since they must hold the machine tightly together.
Any jolts received by the calculator take their toll on the screw posts.
Over time, they're bound to weaken and crack. There are four screw posts in the HP-41, two near the bottom and two just above the battery hatch. These are visible in the photo at right, which is of the front case and keyboard, with the CPU board removed. Notice the two rows of contact fingers between the top screw posts, and the two long rows and one short row between the bottom screw posts. Repairing cracked posts is fairly simple.