Read this from the OS Faq. It will give a foundation to understanding upgrading/downgrading your OS. BlackBerryOS 4.x Download FAQ - UPGRADE DOWNLOADS
What If I Want To Downgrade?
Forum member 'bfrye' made a good guide: Here is a quick install guide to your blackberry. This should easily cover reinstallation of OS and reverting back to previous OS version as well.
Download the version of OS you want to install from your carrier's website.
Extract or install the files from the previous step. They will automatically install into the correct location. This location will be C:\Program Files\Common Files\Research in Motion\Shared\Loader Files. Under this directory you will see directories with the model number of your device (if you installed the correct files) followed by a version number. Under these directories will be a host of .alx and .cod files. Do not modify or delete these files. They are needed to install the new OS.
Assume the BlackBerry Handheld Manager is installed, when you plug your device in, a popup will appear and ask if you want to upgrade your device. Choose upgrade and follow the steps given by the Application Loader. If you have any 3rd party applications you would like to keep, locate and select them at this time to avoid having to reload them after the new OS is installed.
This will also be the time the apploader will ask if you want to erase all data and reload or backup and restore the data on your device. Choose your preference and continue.
Typically a complete OS reload with backup and restore can take anywhere from 5-15 minutes depending on the amount of data on your device to restore.
The apploader will inform you when the process is complete. After this, the device will automatically reset itself (sometime this will take longer than a normal reboot) and once completed and on, your device will be upgraded and ready to go.
Now if you wish to downgrade your device to a previous version, many of the above steps will apply.
First, you will have to remove the newer OS from your PC. Do this by going to add/remove programs in the control panel and removing the software for the new BB OS. Make sure desktop manager is not running when you do this. After complete, restart the Desktop Manager.
When you plug in your device, if everything went well, the Desktop Manager should tell you that your PC has an older version of desktop tools than is installed on the device. It will give you an option to keep the version on your BlackBerry or downgrade to the version on your PC. You will want to select downgrade and let apploader do the work.
Should this method fail, you will have to download the file javaloader.exe from the internet. Place it in a common directory (I usually put mine in the Loader Files directory for easy access. From the directory you have javaloader.exe installed in type "javaloader -usb wipe". This will completely erase all data and applications from your devie. Now navigate to C:\Program Files\Common Files\Research in Motion\AppLoader and delete the file vendor.xml. Now plug in your device and install the new OS from scratch. Open apploader and select the USB port as your connection. It will then bring to you to the main apploader screen and all the OS packages will be listed with status as Install.
If you have question beyond the scope of this document please feel free to post them. However I suggest reading and rereading these steps to ensure you have tried the most basic and tried steps to load or reload your OS. Have fun and enjoy!