Is there a specific reason you need to upgrade? Unless there is something you're trying to fix I wouldn't recommend updating the bios just to get the latest bios. If you need the new bios for features or hardware support or to fix an issue you are having, then I'd update it.
There's instructions on Dell's support page where you download the bios update. Going back to an older bios by downloading the older bios and installing it the same way you install the new bios.