You can take the pipe and cover the stinger and the fitting on top of the pipe
and take the header of. Next fill the pipe with water and put it in the freezer( just like you ar going to make ice cubes) After 15-20minutes keep an eye on the pipe and you will see that the dent will start to come out. Make sure you keep on checking it because if you leave it in the freezer for to much time then you will have a balloon of metal instead of a pipe!!
hope this helps