How Do You Correct an Underbite on a 6 Yr Old?

Was your dentist an orthodontist or a general dentist?

Without further info, it would be impossible to make any recommendation. In general, at age 6, once the permanent molars and front incisors have erupted, the orthodontist would recommend Phase 1 orthodontic treatment to correct anterior crossbite ("underbite"). This would usually involve the use of intra-oral appliances such as rapid palatal expander (RPE) and Delaire facemask. 

 

Following the completion of Phase 1 treatment, the patient is placed on recall until he/she loses all the baby teeth and re-evaluated for Phase 2 (or comprehensive) orthodontic treatment usually at age 12.

It is difficult to answer your question with limited information.  Is the dentist suggesting removal of "baby" teeth or permanent teeth.  Ususally underbites and overbites are not corrected at this age.  The child is growing and is in mixed dentition.  During growth, the growth of the jaws can sometimes be adjusted with orthodontics to prevent teh need for orthognathic surgery in the future, but not always. Every case is different.  Seek the consultation of a board certified orthodontist to help you with your child's issue