Boot into download mode with vol-, home,(if it has it) and power. Press vol+. Connect to PC and open Odin. You should see a blue block with a com number and it will say ...Added in the box. Load the firmware into the AP/PDA slot. (Depending on which version of Odin you use) Hit start. It should reboot when it's finished.
Before you do ANY of the above - be POSITIVE you have a backup of any info you wish to keep that is stored on the device.
Is there a particular reason you want to go back? Did the update break a critical functionality or do you just not like the new look? Samsung offers the theme store to change the look, see if you can find a theme you like there.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Android Central.