Refer to your separation agreement about amendments to the agreement. Most separation agreements allow for the terms to be modified if in writing and both parties’ signatures are notarized. This means both you and the other parent have to agree to the new terms.
Some separation agreements include special modification provisions in the child custody section. Usually this includes a requirement that any changes to child custody must first be mediated. If your separation agreement has this provision, then you’ll need to schedule a mediation date before you can attempt anything else.
If neither party can reach an agreement on changes to the child custody terms, then you will need to file a child custody action in court against the other parent.