The rub, with pastels, is that it isn't very easy to correct your mistakes… There are several methods for minimizing them: this will at least leave you with a surface you can rework.
Even though their ability to adhere is limited, pastels cannot be completely erased. Just one word of advice: Go-light-ly! Applying too much pressure will immediately make the intrusive mark look more pronounced.
Use a paintbrush to lightly go over the afflicted area and sweep away as many particles as possible.
If that isn't enough, rub with a little piece of cotton without pressing down too hard: the color you need to remove will blend into the previous one.
If you have created thick layers if all else fails, use a knife or a utility knife blade to remove as much material as possible. Scratch in the same direction as you drew.
Don't use a conventional eraser. Even a soft eraser would rub the pigments into your paper. The pastellist's best friend is the kneaded eraser or, failing that, a little piece of soft crustless bread.
Use light tapping to remove as much material as possible.
If necessary, sharpen the end of your eraser to a point to remove small details.
Knead it regularly to work the residue into the eraser.
Moisten a cloth with white spirit and gently rub to remove the color. Don't rub too hard or you'll return the paper to its original naked state!