go back on (something)
To not do something one said one would do; to renege on something. People are beginning to worry that the president is going to go back on his campaign promises. You made a commitment to support our project, and now you've gone back on your word!
go back on
Fail to honor or keep, as in You can't go back on your word, or One should never go back on a promise. [Mid-1800s]