If the divorce decree was entered less than 30 days ago, you can file a Notice of Appeal, stating that you wish to challenge the custody order. If it's been more than six months, then you have the ability to file Motion for Modification, if there has been a significant change of circumstances since the custody order was entered. The judge will look at the reasons that he initially gave your ex custody and see if things have changed such that it's in the best interests of the children to either give you partial custody or increased visitation
If it's been between one month and six months, typically, the thing to do would be to look at the reasons that the judge gave custody to your ex and see what you can do to help turn those factors in your favor. If the judge made any negative finding about you, you can try to turn those around. If he didn't, then you can use your visitation to try to show that you are a good father. Make sure that you can show a good relationship with the children, because that will help you.
In some cases, it may also be possible to file a Motion for Relief from Judgment. That usually requires that you are able to show newly discovered evidence that was not available at the time of the original hearing and could not have been found through reasonable diligence or some kind of fraud on the court.
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