You are able to undo previous actions at any time while building a campaign by clicking the Undo button, located on the top right corner of the canvas opposite to the other widgets. Keep in mind once you save the campaign, you cannot undo actions performed prior to saving.
If you chose to undo an action, you are able to redo the action by clicking the Redo button beside the Undo button. Keep in mind once you save the campaign, you cannot redo actions that were undone prior to saving.