On the Mac:
- Quit OmniFocus
- Open this folder: Home > Library > Application Support > OmniFocus
If you are running Mac OS X 10.7 or later the "Home > Library" folder is hidden, so you'll need to hold the "option" key on the keyboard and then select: Menu Bar > Go > Library
- Rename the file "OmniFocus.ofocus" to: "old-OmniFocus.ofocus"
(This way, you'll have a copy stowed away in case you need something out of it.)
- Launch OmniFocus
- From the File menu, choose "Replace Server Database"
- If you have any other devices syncing to this database, you should sync them now and carefully choose the Server option when it warns you of the conflict.
If you also want to undo customization of styles, perspectives, and things like that:
- Quit OmniFocus
- Open this folder: Home > Library > Preferences
(You'll need the Go trick again if you're on 10.7 or later.)
- Rename com.omnigroup.OmniFocus.plist to com.omnigroup.OmniFocus-old.plist
- Relaunch OmniFocus
- Reconfigure your sync settings