How Do I Modify A Child Support Order

I have 2 children who are out of the home and they are with my uncle and wife, but then i have 2 children who are in my home. My aunt&uncle made it plain and clear when we went to court that they needed no money from me and now i am getting child support taking out. My rent alone is $475.00 not to mention all the other bills that i have and i have a 4 year old daughter and 4 month old son as well. i cant even afford diapers because they are taking back and current child support out of my checks. i think my child support should be lowered. i have no money to get anything for the 2 children that i have or myself. not enough money for gas or anything like that...i am not sure what to do anymore my last check was $511.35 and i took home $286.00