If you undo something that is closed, tied, or held together, or if you undo the thing holding it, you loosen or remove the thing holding it. 打开; 解开; 松开
To undo something that has been done means to reverse its effect. 使无效• A heavy-handed approach from the police could undo that good impression. 警察的专制手段能使那个好印象化为乌有。