How do I get my child support reduced

by Joe
(Roy, Utah)

I have 3 total children, my oldest daughter is 18 as of December, 2010. I understand I have to pay till she graduates high school. Which that will be in May, 2011. I have 2 other children with the same mother. It states on the Divorce Decree I pay till 18 years of age or graduates high school wichever is later. how I do go about getting the child support reduced since I will only be responsible for 2 of my children? thank you.