The system hack offered is mostly about faking an UPdate, not reverting. Try at your own risk, but be prepared to bork your system.
Where/how did you download 10.8.3? It is no longer available at the app store.
Technically, you cannot "downgrade" unless you:
a) have a bootable backup from a previous version, and
b) erase the hard drive.
That is is only safe and simple method.
Having said that, IF your Mac came with ML, you can try internet recovery (Command + OPtion + R at bootup or restart) which should reinstall the original OS version, but you also must first wipe the drive. After that, the original ML version should be reinstalled (disclaimer: on some tests, the app store reinstalled the current ML version, so the method is not foolproof).