How do you go about reversing spousal support in WA after the final divorce?

A stipulation was supposed to be in the divorce papers that she would only received spousal support if she went back to school. This was left out, she said she would go and have it changed after the fact, I trusted her and signed so that we could get things finalized. Now she's changed her mind and I'm having to pay spousal support. How do I get this reversed without having to pay an arm and leg in legal fees? Thank you!