Step by Step Guide to Downgrading your BlackBerry OS
Uninstall all the versions of the programs that you do not want and any versions that are later than the downgrade. To uninstall the programs you need to go to the start menu and look in your control panel for the “Uninstall Program" or “Add/Remove Program" option, depending on the version of Windows you are using. You'll then be presented with the list of operating systems that are installed. The best way to figure out which one you want to delete is to go by the installation date. If this doesn't work for you, you can always uninstall all the operating systems and then reinstall only the one you want.
You'll need to download and install the latest version of the BlackBerry Desktop Manager Software.
If you haven't already done this, you'll need to backup your data and any third party applications that you want to keep.
Download JL_Cmder to help you wipe your phone completely clean. This is the best option to ensure that everything from the previous operating system's installation and use is completely removed. This will wipe everything from your phone and you will see a white screen that reads “Error 507."
Now it's time to load the Blackberry Operating System that you want. You'll need to start the Desktop Manager software. Choose the Application Loader. Choose the “Add/Remove Program" option. Check the boxes next to all of the programs you want on the phone, such as the Blackberry Messenger, etc.
Click “Next." You'll get a confirmation that the programs are about to be installed. Now you can sit back and wait while all the programs you selected are being installed.
All that's left to do is to restore the backup data.
You have successfully downgraded your operating system.