While some tablets will allow you do downgrade from a higher Android version most phones will not. Carriers have based their customer no service on a cookie cutter design where they can offer the same advice to people with the same device. That way they don't have to deal with different versions of the same device. You would most likely have to root the device to downgrade it and to go back multiple versions would definitely require root.
IMO the best place to have root questions answered is at XDA Developers Forums. While we do have a root and developer community here it is much more robust at XDA. You would likely be able to find a complete how to for your device there, but you will probably want to register an account there to get the best information, help and advice.