Given the length of the battery wires inside a Belkin F6C1500 UPS, you might think any arrangement will work. Not so. The wires from the guts of the UPS must exit to the batteries exactly like this:
There’s a black wire tucked under the red wire, both of which must exit though the angled slot and run toward the front of the battery compartment.
Seen from the front, the red wire connects the positive terminal of the lower (left) battery to the negative terminal of the top (right) battery and the black wire connects the negative terminal of the lower battery to the UPS circuitry:
Trust me on this: there is no other arrangement of those wires that will simultaneously connect everything properly and fit within the case.
As for disassembly, the small tab on the left end of the case holds the front panel in place. Press that inward with a flat screwdriver, then slide the cover toward the tab. Four locking slots along the sides will disengage and you can then lift the panel off.
With that out of the way, there’s a screw hidden under the BELKIN label in the middle of the removable cover: