Can I undo a parent page selection?

There is not necessarily an "undo" option for parent page selections. 

 

However, if you wish to remove a parent page from a page simply click the Home Page option at the top of the list and the page will be moved back out to the main level.

 

To change a parent page selection, simply click the name of the desired parent page from the list.

 

As soon as you click a page name, that page becomes the parent.  Saving is not necessary since the change is committed instantly, although it is recommended.