How to re-shape a bent steel fender
Generally you'd take the fender off a bike, put it on an anvil, find the right place on the horn of the anvil that matches the radius of the curve, and bang out the ding. Then you'd buff out any scratches or rechrome it. 



(this anvil's horn is upward curving, you'd want one with a downward curve that matches your fender angle more).

You don't need a big huge anvil, there are hobby anvils that are much smaller. You do want to choose the right type of hammer (most likely bronze faced) so that you don't scratch or mar the finish too much.

You can  also take it to an autobody shop or a metalworker and have them do it for you.