Can I get my child support payment adjusted after the divorce has been final for two years? I live in Los Angeles.

My ex made me sign a letter that he did not have to pay me my child support because I was responsible for 50% of our IRS bill and they swooped his account. I have received no payments from him in two years and have had to borrow money from him for rent which he then makes me sign more child support away. I should add that my payments were very low to begin with because all of the money he has he inherited four months after we separated. Recently I had to leave my daughter with him because I could not afford to feed her. He does have income from social security, his dad's pension and from the investments he made with his inheritance. Can I change this legally?