I know Vodafone have an update for the Arc (not S) as the Desktop software keeps telling me there's an update available for my phone!, but As I've customized mine (Rooted with a generic ICS and unlocked the Bootloader)
A couple of things you could try:
Google for Sony SEUS (Sony Ericsson Update Service) and try and update the phone with that utility (it will most likely wipe the phone so backup everything!!).
Flash the Generic ICS ROM found on XDA Forums (loads of guides on how).
Root and Unlock the Bootloader and Flash with a Custom ROM (again from XDA)
Or wait till your carrier gets off it's Fat Arse and releases the new Firmware 'Fettled' for their phones!
Try the Sony PC Companion first, from http://www.sonymobile.com/gb/tools/pc-companion/, Plug your phone in and it should tell you if there is an update available, and then prompt you to install it (this option will back up your phone for you!).
OR if that fails...
Try the Sony Ericsson Update Service (SEUS) from http://www.sonymobile.com/global-en/tools/update-service/, and again plug your phone in and follow the prompts... it takes a little longer, and wipes your phone (at least it did when I used it at one point!) so you need to manually backup your phone (I find letting Google take the strain on this works wonders!!).
If they fail then it's over to waiting for '3' to get the update released, or flash the phone with the 'Generic' ICS ROM (which I ended up doing as waiting for Vodaclone was taking for ever!!)