Chin surgery has two directions:  to enlarge or to decrease the chin.

When is too prominent and / or  too long , it reduces the  bone.
The surgery is done either under general anesthesia (preferred) or under local anesthesia and lasts an hour. The pain is minimum or absent. But the swelling can persist several weeks. In addition it needs time to accommodate with the new skeleton. 
Interesting is that after the mentoplasty the lower lip may look slightly raised ! 

When the chin is too small, placed too back, we have two ways of solving it.
 A silicone implant it will be introduced between the bone and muscle. Implants have different sizes and are shaped to fit to different  clinical situations. A new tissue pocket  will be created where it will be place it the implant. The surgery is performed under local anesthesia and lasts less then 1 hour. The healing process is fast , with no pain.
 Another solution is to section the mandibular bone and to fix the dislocated fragment with screws. The procedure is more painful and the recovery is longer.
Find the best solution for you along with your physician.

This procedure is often associated with the correction of nose, and together are referred to profiloplasty.

In these pictures it is a case of chin reduction.