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Someone in your situation wants to file a modification of custody petition - see HERE. This asks the Court to change the custody situation.
NOTE that just because the child wants to change custody does not mean that the Court will automatically agree. While the Court places emphasis on an older child's preference, it is still one of many subjective factors the Court will consider such as general stability of the parent, financial stability, indoctrination of the child in the current school and environment, household condition and living condition of the child, other persons living in the house, etc.
Of course, if the ex is in agreement, then the Court will normally sign off on this without any issue. It only becomes a contested matter if the ex is not in agreement, forcing the Judge to make a decision.
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