It depends on age and where you are at. My parents looked into this for my sister when she told them to f-off and moved out. It really didn't involve much more than filing a bunch of forms at the county courthouse and in their case a discussion in a judge's quarters explaining the situation and signing some forms after he agreed.
So, first step would be to go to your county/city courthouse and talk to them about it, and then probably find a lawyer so that you are certain all the paperwork is right. But, if they are younger than 16 or so, don't count on being able to do it. Even then there are some hoops to jump through, thus the lawyer.