Stick it in the freezer. That will reduce the adhesion of the bed heater significantly and theoretically allow you to peel it off. Go slow, wear gloves, and do not force it. There are thin copper wires inside, if you pull them too hard the wires break and you buy a new bed heater. the top PEI surface should come off the same way.
If you find you absolutly cannot remove it by pulling even with the freezer trick, you can use a citrus based glue remover to remove the heater. This will leave a residue that you absolutly MUST remove from the heater before attempting to re-attach it. While normally not the best idea for cleaning rubber, several cleanings with a glass cleaner containing ammonia such as windex will probably remove the oils. That may damage the rubber over time, but at that point you probably don’t have much choice. You would then wash the surface with isopropyl alchohol several times to remove any remaining windex residue, then use a high temperature contact cement to re-apply the silicone rubber heater.