If the dent goes into the rim you can use an adjustable wrench to pull it out some, if the dent pushes out you can use a block of wood and a hammer, set the flat side of the rim on a workbench edge or table edge, set a block of wood against the rim and use light taps to flatten. It will never be perfect due to the metal stretching but you can get it to the point that you will only feel it while braking.