How do you fix and underbite?

Depends... A bad bite can be caused by a problem with skeletal or dental alignment (or a combination of both). Dental alignment problems can be resolved with braces (sometimes you may need some teeth out). Skeletal problems need braces and surgery. See an orthodontist to see which category you fall into.
See Below. 'underbite' and 'overbite' refer to a misalignment or malocclusion of the teeth of the maxilla (upper jaw) and the mandible (lower jaw). Sometimes the underlying bones are misaligned (a 'skeletal' malocclusion). A skeletal malocclusion can require orthognathic surgery for correction. In some cases, orthodontics is all that is needed. An orthodontist can determine the correct choice.
Braces and Surgery. Underbites vary in cause and severity. Underbites that are due to abnormal development of the jaw bones can only be corrected surgically while minor underbites can often be corrected with braces or invisalign. See a competent orthodontist who can properly evaluate the cause and extent of the problem and recommend the correct solution.
It depends. So many factors go into answering your question, including the severity of the cross-bite, your appearance and desired change in appearance, etc. Many times orthodontics can help, but often orthognathic surgery is the answer. Consult with an oral surgeon and orthodontist who together can work to correct your smile.
Depends on cause. An underbite (lower teeth in front of upper teeth) may be caused by tooth malrelationship, jaw malrelationship, or most probably both. Severity of malrelationsips determine treatment choices of tooth realignment (braces), jaw repositioning (surgery), or probably both. Know that there may be other malrelationships as well as the underbite. Please see fully qualified Orthodontic Specialist.
Forces. Orthodontics is a science of using subtle forces to move teeth to create a beatiful and healthy smile and bite. These forces can rotate crooked teeth and adjust your bite to correct your underbite. Absolutely can be addressed-- we do this every day!
See an Orthodontist. Depending on the severity of your underbite, braces alone may be able to fix it. If it is too severe then jaw surgery may be the only way to correct your malocclusion.