Just peel it off and use an adhesive remover to clean up the mess.

 

Be sure not to drip any on the dash though

 

i used to do it for my dealership and highly recomend not messing with the rear window because of the defroster. if you don't use the correct adhesive remover, you could destroy you defroster! side windows aren't very hard, you can peel them off, use regular adhesive remover and a one sided blade, and no the blade will not scratch your glass. good luck!