Start the reassembly, for the most part it involves reversing the disassembly steps but there are some things that need to be noted.

Drop the battery cover clip back into the bottom of the case in the orientation shown in the previous step.

Use a small flat blade screwdriver to pull the spring back and seat the tip of the spring behind the notch in the battery cover clip. Carefully close and open the battery cover a few times to make sure that it is now staying closed properly.

Reinsert the lower PCB, taking care to slide the back right corner in first, the screw that was loosened but not removed in step 5 holds a metal clip, the edge of the lower PCB has to slide into this clip.

Push the left side of the lower PCB back until is can slide down over the vertical plastic alignment guide on the left edge of the case.

Use a flat blade screw driver to get the lower PCB back under the retaining clip at the left front in a similar way to how the board was removed, don't try to just push down on the board, the fit is so tight the board might crack.

Make sure the lower PCB ribbon cable clip is in the up / released position, reinsert the lower PCB ribbon cable guiding it into position with a needle nose pliers, tweezers or forceps, be careful not to damage the cable and make sure that it is in far enough that only a tiny stripe of the ribbon contacts remains visible.

Rotate the black retaining clip down to lock the ribbon in position. Reinsert and tighten all of the silver screws that hold down the lower PCB.