This info was already posted for you above. You need to install this software linked in [url=http://www.blackberryforums.com/general-8300-series-discussion/89908-curve-tmo.html#post628940]this
TMobile doesn't offer software for the Curve because they haven't released the Curve for TMobile.
To downgrade your OS, follow these instructions in this order:
1. Uninstall any newer OS's from your PC using Control Panel, Add/Remove Programs.
2. Download and install Deskotp Manager 4.2 SP2 from here
3. Download and install to your PC, the O2 OS (from the link above). Make sure the BB is not plugged in during this installation.
4. Delete the vendor.xml file located in c:\program files\common files\research in motion\apploader
5. Plug in BB. Open Desktop Manager and run Application Loader. It should show that a "downgrade" is pending.
6. Finish the installation.