If the parent does not agree that you should be managing conservator, it can be a little harder. You must prove that circumstances have changed and living with you is in the child’s best interest.
A court would be more likely to appoint you as a managing conservator if the parent has voluntarily given the child to you or another person or agency.
Contact an attorney if you want to be the managing conservator. An attorney can help you file a motion to modify the court order. Read more about motions to modify here. Check with your local library or law library to find forms you need to file.