You cannot undo what has already been done.
I can't undo it.
The "undo" option reverts the last change done to the document.
He did not want to know, but you cannot undo knowledge.
David's mother had been developing a pill to undo the effects of the operation.
Could you help me undo the buttons?
Please undo the package.
Don't undo your bootlaces until you have seen the river.