Cut the old ones off, or they will just crumble off. They are on a stud/spacer.
The back one has a 1/4" bolt with that spacer - don't throw the spacers away
Remove.
This will loosen up the mounts for the front ones. While getting the old ones off is easy, even if done correctly, it is still a little squeeze to get them back in place with the new ones. Anyway, don't try to remove the stud/spacer. You have to move the sub chassis forward and/or up. I give you this choice depending on what you want to stretch on the bottom side - mostly wires.
If you can get the chassis forward, then slip on the new rubber bumpers and slide it back through the little cut-outs in the main-chassis/mount. It will still be tight. You may need to widen the space that is forward of the normal resting position (stretch the chassis apart). Then slide the sub back in.
The other alternative is to take the stretching o f the main chassis to a greater extreme to allow the sub to drop down from the top. This can usually be done without bending something - permanently.
You can also do a combo and slide the chassis forward as far as you can and then stuff the new rubber into place. Most of the time this is not as impossible as it sounds.
The back one is easy to put back in place.
So basically you are somewhat at the mercy of the wires underneath. These (hum, they seem to be either all yellow or black) have become brittle over the years and stretching and twisting them can generate a lot of crumbling.
Go easy.
P.S. I haven't done one of these this way for ages. I had to go look at my Faux Faux 37-620 for a minute.
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/