To modify the amount of child support (either  an increase or decrease in the amount ordered), one of two things must be proven in court: either (a) you show that the circumstances of the child or a person affected by the order have materially and substantially changed since the date the order was signed; or (b) it has been three years since the order was signed and the monthly amount of the child support award under the order differs by either 20 percent or $100 from the amount that would be awarded in accordance with the child support guidelines. Most cases will fall under the three-year category, so the question of whether child support can be modified becomes primarily a question of math.