You'll need desktop manager. See

BlackBerry Desktop Manager Downloads - BlackBerryFAQ

The latest version is 4.7, I believe. If you are using an older version of Outlook, you will want to download and install version 4.2 SP2 (or SP3, if there is one). There are versions with and without media manager. You don't need media manager, and the version without will be a smaller download.

Here's another link to OS downloads.

BlackBerry Operating System Downloads - BlackBerryFAQ

Remember, you can use an OS downloaded from any carrier. Only condition is that it must be the OS for your model blackberry.

Download and install desktop manager (the desktop software) on the computer. Make sure desktop manager is closed. Download and install the OS (device software) on the computer. Download and install JL_CMDER. (See links in post above.)

Follow the instructions at the FAQ to wipe the blackberry and then install the operating system. (See links in post above.)