The Undo command is found at the top of the Edit menu. Virtually any edit you make to a plan can be reversed using the Undo command.  You can undo up to the last 40 edits and Undo only applies to edits made since the current plan was 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 symbols in your plan and use the Delete Symbols command, you must use the undo command 40 times, and 10 symbols will remain deleted. Use these global commands with caution.