The BBFAQ has a simple guide. Upgrading is the same as just reloading. How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ
The latest OS version is available from a link in a sticky thread at the top of the 9600 forum. No need to worry about what carrier released the version of the OS you install. As long as you remember the step to delete the vendor.XML file it's all good.
Warning: this will delete all info on your device. You will need to restore your data from a backup after reloading the OS.