If you lose your BlackBerry Messenger during a software update, it can be retreived. The BlackBerry Desktop Manager holds all of the stock programs that come with the BlackBerry, and they can be reinstalled to the phone at any time. Even if you want to delete the program from the phone to conserve space, you can put it back on at a later time.
Download the latest version of the Blackberry Desktop Manager if you do not already have it (see Resources).
Open the software and connect your phone to the computer using the USB cable.
Click "Applications" on the left and wait for the available applications to load.
Scroll to "BlackBerry Messenger." Click the plus-sign button to the right of it. This places it in the "Pending" section.
Click "Apply" to install it on the phone. The phone will reboot when the installation is complete. Do not disconnect your phone from the computer until then.