Undoing and Redoing Changes
All Office applications have Undo features. If you make a mistake, click the Undo icon, or choose Edit, Undo, or press Ctrl+Z. Every Office application supports at least one level of Undo, and some enable you to undo a number of successive changes. If you discover you made a mistake five minutes ago, you might be able to recover by clicking the Undo button repeatedly. If you close your file, however, all bets are off—all Office applications clear the Undo history when you close the document.
Word and Outlook have a virtually unlimited number of Undo levels. As long as you don't close the document, you can undo anything you've done. (There are some physical limitations to the size of the Undo file, but in practice they aren't significant.)
Word and Outlook's tremendously powerful Undo capability enables you to bring back material that you might have thought was lost. For example, if you're working on a speech and you decide the opening paragraph you started with is better than the one you ended with, you can easily restore it. First, save your current document! If anything goes wrong while using Undo in this way, you can exit without saving and reopen your document to start over.
Tip from 
Click the drop-down Undo list and scroll all the way to the bottom, selecting every action on the list. When you release the mouse button, Word undoes everything you've done in the current session, restoring your document to the state it was in when you first opened it. Next, select the text you want to restore and copy it to the Clipboard (do not, under any circumstances, use the Cut command). Now scroll to the bottom of the drop-down Redo list and click to redo every action you just undid. Your document is now back to the state it was in before you performed the multiple-level undo, and you're free to paste in the paragraph from the Clipboard.
If you've lost the ability to Redo changes in Word, the Troubleshooting tip
"Cutting Text Clears the Redo List"
in the
"Troubleshooting"
section near the end of this chapter.
Excel, on the other hand, limits you to 16 levels of Undo. This relatively severe limitation has been part of Excel for years, since the Undo feature was first introduced.
→ To overcome this limitation, you need to hack the Registry; see "Customizing Excel".
PowerPoint enables you to select the number of levels of Undo you want to support. (Choose Tools, Options, click the Edit tab, and spin the Maximum Number of Undos box.) The default value is 20, but you can increase this to a maximum of 150.