Go to the "Options" setting of your BlackBerry's home menu.
Click on "Security Options" then click the "General Settings" line to open up the settings panel.
Click on the BlackBerry icon button to bring up a list of choices. Select the option to "Wipe Handheld."
Confirm that you want to delete the handheld in the box that appears.
Enter the word "BlackBerry" when prompted to then hit OK. This restores the BlackBerry OS.