What Carrier are you on??
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!
(
http://forum.xda-developers.com/forumdisplay.php?f=962)