Heat gun, 3M adhesive remover and time.
Start with the heat gun and apply heat slowly and do a test pull as you go. If the decal pulls up but leaves a glue residue behind, use a little more heat until you get the decal to pull with the glue. Just be careful with the heat gun and heat slowly and do not over heat the area.
Once you got the decals off, go back over with the 3M adhesive remover to ensure all the glue left behind is completely gone. Also remember to have a beer or a rum / coke beside you and ready at all times, very important!