My x & I decided abut a year ago it was time to up my child support,didit ourselvs n now need it legaly binded?

we are looking for the least expensive and quick way to get it legaly changed or i could be screwed out of $160 a month. The x will soon be going through another divorce with more kids to support. The soon t be x wife thinks its already legaly binding. If we d not make it so then thats more money she will get from him and take from my son. Please help