It makes no difference what carrier released the OS. You can load it on any Pearl.
Make sure the latest Desktop Manager is installed. Currently 4.2 SP1
Download the above linked OS. Then double click on the file you downloaded to install it to the PC.
Delete the vendor.xml file located in c:\program files\common files\research in motion\apploader
Plug in BB, open Desktop Manager and it should prompt you to upgrade.
The installation will appear to hang for up to 20 minutes at times with a spinning hourglass....be patient, and let it go.