First of all, if you need to keep your rules, objects and so on, DO NOT DO THIS. Both of these methods will wipe out everything you currently have on the system.
I would just reinstall the OS. If it's a VM or open server, you can feed it the ISO image of the version you want.
For an appliance, you should get the ISO image of the version and feed that to ISOmorphic to make a USB installation drive. You then boot the appliance from that, and it overwrites whatever is installed. ISOmorphic can work for open servers, but some don't like to boot from USB for whatever reason.