This is easy. The back of a d5 just pops off and the speed dial is free to slide out. If the dial is just snapped at the bottom, as is common, then new ones are available from frozen cpu. Alternatively you can just use a small screwdriver to adjust the speed after removing the red dial. The bottom of the red dial is just a fragile x shape that fits into the control pot.
You may need to gently remove the remains of the dial from the control pot.
If you have any problems like the damage is more severe then there are other options.Edited by Jakusonfire - 4/6/14 at 4:23pm