Removing the back caps is easy, removing the four screws around the edges will allow the cap to come off, sometimes they may be stuck and will take a slight tap with the handle of screwdriver to crack loose.
The cap will be loose but still attached to the diaphragm with the connection wires.
You will need to disconnect the wire from the diaphragm terminals, here's where you need to be careful, loosen the screws enough to remove the wire terminals.
Most likely your going to find the foam has turned to sticky mush, hopefully pieces of it have not fallen of and got stuck all over the diaphragm. You can order new foams from JBL parts, there actually called "dampener pads" the part number is 61286. With some good open cell foam you could make your own and glue them in.
While your in the driver it would not hurt to remove the diaphragm and clean the voice coil gap in the drivers top plate. This is were you really need to be careful, after removing the three screws holding the diaphragm
the diaphragm should lift off the driver, there are two centering pins in the driver that stick up about an 1/8 of an inch that fit into holes on the diaphragm to keep it centered, sometimes the diaphragm will be stuck on those pins and you will need to work the diaphragm off the pins by starting a thin screwdriver or even a knife blade around the edges of the diaphragm.
Clean the gap out by running a piece of heavy tape around the gaps inside and outside edge, blowing out with compressed air and then repeat again. Look in the gap with a bright light or outside in the sun for any bits of dirt.
Carefully reinstall the diaphragm over the pins and start the screws back in. Normally when reinstalling a diaphragm you will run a test tone into the diaphragm to listen for buzzing and overtones. Something around 400 hz will work at lower volume level yet easy to hear. Slightly shifting the diaphragm or even the pattern that you snug the screws back down can make a difference getting rid of any buzz or overtone.
You will have to get creative in connecting your amp the diaphragm for the tone test. You can download
sweep tone generator apps from many different places
Do not use a screwdriver that has removable tips!!