Each time you save a presentation, you can automatically retain a copy of the last saved version. That way, if you change your mind about edits you made, you can go back to (revert to) the backup version of the presentation.
The best way to create backup versions depending on which version of OS X you’re using. OS X Lion (v10.7) and later automatically saves a snapshot of your presentation every time you save. You can access an archive of all previous saved versions at any time. To learn about accessing and using past document versions in Lion, see Finding an Archived Version of a Presentation.
If you’re using Mac OS X v10.6 (Snow Leopard) or earlier, you can set up Keynote to automatically create a copy of the last saved version of your presentation. You may also find this useful if you’re using Lion, and you want to save a backup version of your presentation on another hard disk on your network.
Choose File > “Save a Version,” or press Command-S.
Choose Keynote > Preferences, click General, and then select “Back up previous version.”
The next time you save your presentation, a backup version is created in the same location, with “Backup of” preceding the filename. Only one version—the last saved version—is backed up. Every time you save the presentation, the old backup file is replaced with the new backup file.