I'll give it a go ......... but I'm no expert.
Modern plastic bumpers are molded and have an inbuilt memory. I got a dent (which left a pronounced crease) in my other car (Toyota) and a panel beater mate advised me to put it out in the sun on a really hot day and leave it out all day. 99% of the dent came out by itself. If the dent is concaved (past the point of returning by itself) then it will need encouraging to return it to what shape it was then follow the above. With a grin on his face he then told me that he would charge the customer for his time and skill level for doing it.
To get the last little bit out, use a hair dryer, (not a hot air gun) but be real carefull not to heat it hotter than you can put your hand on it for some time.
Anyway mine worked a treat and with a cut and polish you would never know there was a dent there.
Red