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The latest is BlackBerry Handheld Software v126.96.36.1996 (Multilanguage) for ATT.
Go to the RIM site BlackBerry - Support & Services
then to downloads and follow the instructions. I also highly recommend you download desktop manager and do the update from a pc. DM is free from RIM also.
make sure your PC is powered and let it finish. Also make a backup using DM before you do the update.