Take the display board off the wall, and lay it down on a flat surface.
Locate the dent, or if there is more than one, choose one to work on first.
Make a slit in the edge of the foam display board next to the dent. The slit only needs to be as long as the dent.
Rip a sheet of cotton into small pieces, or pull apart some cotton balls.
Slide a butter knife into the slit, and use it to lift the top surface of the board away from the foam.
Push the cotton into the dent, in small pieces, until the dent is looks flat from the front.
Remove the butter knife and check that the board looks flat. If the dent is now raised, remove some of the cotton.
Put a small amount of glue onto the butter knife, and glue the top surface back onto the foam.
Glue the outer slit closed, and allow to dry.