Changing custody

Once a divorce is final, and a custody schedule is set, what is the process for changing it? If, for example, there is an agreement of 50/50 and the schedule dictates Monday-Monday, but it is easier for the parents to do 2/2/5. What is that process?

If you and your ex agree, the schedule can be changed by mutual consent.
If not the party who wishes to change the schedule must prove that there has been a substantial change in circumstances which affects the best interests of the children and warrants a change in the schedule.

Was the custody schedule set by a separation agreement or is it part of a court order?