How do you fix a cross bite?

See a dentist. It depends on where the cross bite is. If it is in the back, the dentist may choose to use some kind of an expander. If it is your front teeth, they may use something called a sagittal. There are also other things that can be done. You would need to see a dentist for the best course of action.
1st determine cause. If problem is dental, tooth movement and/or restoration(s) can adjust tooth positions. If problem is skeletal, jaw movement required. Please see a cross bite expert, a fully qualified Orthodontic Specialist, to do a differential diagnosis, determine the appropriate course of treatment, and then choose the appropriate cross bite correction appliance.
Orthodontist. There are numerous ways. Placing a crown might fix a simple crossbite, but more severe crossbites require braces and surgery to fix. Go visit a dentist or orthodontist to find out the best options for your teeth!
Depends. Depending on the severity and/or the cause, orthodontics can often be used. In certain circumstances surgery is needed.