There are many tips online Alan. Some I wouldn't want to use such as petrol or a solvent such as white spirit.
In my experience they are very difficult to remove, warming up with a hairdryer and using an old credit card has worked in the past, but be prepared for a battle and allocate some time if you want to remove them completely; depending on adhesion properties.
I am in the same position, but these holders are so good I will be battling with mine in due course. Let me know how you get on please and if you happen on a better technique.