You can try one of two tings that tend to work for me.
First I would try heating up the backplate with a hair dryer or something similar. Once it warms up you should be able to peel it away from the board slowly.
You can also try getting some alcohol behind the backplate, as it wont damage the motherboard but may loosen the adhesive.
Though some manuals tell you to adhere the plate, most mounting pressure will hold it in place just fine, then you dont have to worry about it later.