Hello pj and welcome to BB Forums.
There are many excellent Sticky Thread FAQs at the top of the General BlackBerry Discussion forum with tons of information.
Included is a list of web sites, by carrier, where you can download the BlackBerry Handheld OS from. Here is the thread: BlackBerryOS 4.x Download FAQ - UPGRADE DOWNLOADS
Anticipating the next set of questions, here are the steps required to install/upgrade the OS:
- Shut down Desktop Manager
- Download the handheld OS for your BlackBerry
- Execute the program you just downloaded
- Locate and delete VENDOR.XML on your PC C: drive
- Connect the BlackBerry and Restart Desktop Manager