The lower PCB routes the 9v and the guitar's signal. It also has the switch on it. The PCB connects to the upper PCB via a ribbon cable of 6 wires. The white plastic connector at the end of that cable will detach if you jam a knife blade or a fingernail into the base of the connector where its male/female ends meet.

Some re-housing tutorials recommend that you toss the lower PCB. That's unnecessary, unless you just like to solder. The lower PCB gets the 9 volts into shape (so to speak), and were you to discard it, you'd then have to re-construct another circuit to do that. It's easiest just to re-use the lower PCB.