Make sure it is not a tumor carrying pain in it. If it is just a lump then you can get rid of it by mixing colloidal oatmeal with water and soak cloth in it and keep it for few minutes on its lump till it softens. It will come off then otherwise use Hydrocortisone ointment that will heal the area soon. Some times it becomes important to remove lump through surgery if it starts bothering your pet.