Use the Undo tool to cancel the most recent action or a series of recent actions.
To undo a single action
Undo panel 
(Undo). Text notes do not have the scroll down feature for multiple undo actions. You can click Undo repeatedly to step backward through edits.
Revit LT cancels the most recent action.
To undo multiple actions:
Revit LT cancels all actions up to and including the selected action.