I didn't uninstall and everything went fine.But Javacool recommends uninstalling first.
Quote:
Upgrade Instructions
It is recommended that you uninstall your current version of SpywareBlaster before installing the new version. To do this:
1.) Open SpywareBlaster and press the "Disable All Protection" link under "Quick Tasks".
2.) Close SpywareBlaster.
3.) Go to Add/Remove Programs and uninstall the entry named "SpywareBlaster v3.5.1"
4.) Download the latest SpywareBlaster installer from the link above and run it.
5.) Enjoy the new version of SpywareBlaster