What is Undo ?
Undo is verb 1. to unfasten some- thing which is tied or buttoned The first thing he did on getting home was to undo his tie. Undo your top button if your collar is too tight. 2. to upset the good effect of some- thing His off-the-cuff remarks undid all the good work done to increase racial coopera- tion. (NOTE: undid – has undone )
source: Easier English, Student Dictionary Upper Intermediate Level