What do you mean by "really stuck"?
Does it budge at all when you try and move it? I would just pull back on the top of the backplate with a constant force, and wait. Sometimes the glue will slowly weaken and break.
If you can't get it off with with force alone (or fear breaking your motherboard) get a cleaning solvent designed to remove sticker residue from plastics. This way the solvent won't damage the motherboard and will allow you to remove the backplate rather easily.