Rabbits generally will keep themselves clean, but sometimes their fur may get matted, tangled (especially if its a very furry or wooly breed!) or all over the place when molting.
Usually in cases like this, a good wide-toothed steel brush should do the trick. Brush down the shoulders and the sides of the rabbit while holding it down securely. Sometimes, you may want to start by using your damp hands and stroke your rabbit down from head to tail to remove the dead, loose fur laying on its coat before starting to comb.
To brush the tummy and undersides, turn the rabbit around on its back, still holding it securely in your hands, and brush the tummy down.
If you run into matted fur, DO NOT try to force the comb through it. Use your fingers to tear it apart instead.
Also, do not brush or blow dry the rabbit’s ears.