Undo and Redo can be accessed from the Edit menu, as well as a list of the last 10 operations that were performed. Selecting an operation from the list undoes all operations performed after the selected operation (including the selected operation). The undo / redo operations are specific to each tab.
The Undo levels may be set anywhere between 2 and 100 under the General tab in the Preferences. The default is 50.