Difficulty: Moderately Easy
Step1
Wait until the underage party becomes of age and renew the vows. Apply for a valid marriage license. Have a new ceremony with a justice of the peace or a clergyman. Have witnesses present and redo the entire marriage. Get permission for the parents of the underage person.
Step2
Get divorced and then marry the right person. An invalid marriage because of a previous marriage can be validated by wiping the slate clean with the courthouse and county records office. Wait the appropriate amount of time and then marry the person from which the invalid marriage arose. Consult the county clerks office for special rules in your area.
Step3
Wait until the party without mental capacity regains capacity. This is the same capacity needed to make a will or enter into an agreement. The capacity only needs to be present for an instant of a second. If the person has periods of on and off memory, wait until their mental faculties are present and get consent then.
Step4
Wait until the barrier to marriage is removed and then re-do the marriage. Go through the process to invalidate the marriage with the court system. Get an annulment or divorce and then start all over. Follow the marriage rules of your state.