I assume it’s locked? Otherwise... just pull it..? If locked, and if it’s like the front, there will be a sort of gear on both sides, and some parts that engage the gear to lock the belt. I think I see it, the horizontal bar in the first picture. There is a little bit of play where you can get a small screwdriver in there to manipulate the teeth from engaging so you can pull some out. It will probably be very slow and tedious until you have enough out that you can grab, feed in to release the lock, and slowly pull back out.