How do we go about getting custodial parent change?

While your daughter is a "child attending school," that term only applies to the duty to pay child support, not custodial status. The good news is that when she turned 18, she became an adult in her own right; she can join the Army, get married and vote and the original order granting her mother sole custody became obsolete. While we do not know what amounts of child support you have been directed to pay, you could seek a modification of child support in the court that issued the custody order. If she has been living with her mother but wants to live with you while she attending school, she can move to your residence and then you and/or her can bring a motion to modify child support. If she is living in your house while she is going to school, the court would probably order her mother to pay her child support directly as opposed to paying it to you. If your daughter is going to school away from both parent's homes, both parents would be ordered to pay an appropriate amount directly to your daughter until she reaches age 21 or is no longer in school.