Gently pull them away and try to stick something behind them to help pull them (plastic scraper etc that won't damage anything if it slips). A length of string works as someone already mentioned - use it to slice through the sticky pads.
Once you've got them off, use white spirit to clean up the area and remove all the left over bits of sticky pads.
Places like Halfords sell number plate adhesive strips. They're double sided foam strips - normally use 3 strips, one each end and one in the middle.
Make sure you clean the area well with white spirit on both the car and also on the back of the new number plates to ensure you get a good stick with the new pads.
The idea of glue to hold number plates is that you cannot (or should not be able to) remove the plates intact.
This is supposed to prevent plate theft and subsequent legal troubles for the registered owner.
If the foam double sided stuff is holding your plates on dental floss is a very good high tensile string with which to try and remove them, beware that peeling the remains off your plates may pull the yellow or white lamination off with it.
Carpet tape looses it stickiness over a short period and the plate will fall off, use the proper sticky foam pads from halfords or any good Motor factors.. and make sure you mark them out properly before fitting.. measure twice, fit once..
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