Download and install Desktop Manager to your pc: BlackBerry - BlackBerry Desktop Software
Then download the file Penguin3107 pointed you to.
Double-click the installer for that program and install it onto your computer.
Connect your BlackBerry to your pc, re-open Desktop Manager, and update/install the OS onto your BlackBerry. Follow these instructions step by step: How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ