The Undo command is found at the top of the Edit menu.  Any edit made to a plan can be reversed by the Undo command.  This command can only reverse the last 40 edits.  Undo only applies to edits made since the plan was most recently opened.  When you use the Save command, the Undo buffer is cleared, so nothing done prior to the last save can be undone.

Note:  When Undo is used on global delete commands, it cannot undo the global deletion, but rather the individual deletions that were caused by the global undo command.  For example, you have 50 icons in your plan and use the Delete Icons command, you must use the undo command 40 times, and 10 icons are forever deleted.  Use these global commands with extreme caution.