Menu: Edit - Undo Hot key: U Whenever you did an action and wish to undo it you can use this function. It's possible to take back at least the last 200 changes made on the drawing. Note: However, you can't undo these actions: adding, removing and renaming layers.
7.2 Redo
Menu: Edit - Redo Hot key: Shift-U Undone actions can get restored with this function.