Hot iron on low heat with a towel between iron and bumper. Takes a while and you really have to press down without actually melting the bumber material, but I have done this on my car and it worked. Go to a second hand store and find an ancient iron without the steam holes if you can, because that is the one problem I had is the lack of heat in the holes left a funky pattern for a few weeks. Hope this helps and I think that maybe others have used different heating methods as well.
Jed Clampett
(couple of Fiat's, nothing fancy)