Use a hair drier to warm them up, start at one corner and peel the decal,
As you heat the decal, too much heat will make it stretch thin and break, not enough heat will make it hard to lift, You have to practice, after you get the decal removed, you need Rapid Remover to get the glue off. Here is a link, this stuff smells like oranges.
And as a side note, keep the bottle handy, this stuff removes a LOT of stains and marks on items that are hard to get off, like sharpy marks, shoe scuffs etc. whenever I am fighting marks on walls or anyplace, I grab Rapid Remover and it many times handles the job.
You know that glue on the new toaster oven that was left on after you removed the warning sticker? Comes right off, Stuff is expensive though.