To replace an old/ empty roll in a cartridge, simply pry the spool arm away from the roll slowly but firmly. It should just then pop out.
Once the spool arm is detached, the spool and the stabilizer can be disassembled as well.
To reassemble and install a new label roll, make sure you align the slot on the stabilizer to the pin of the spool. This will prevent the stabilizer from moving and will lock the stabilizer in place.