Keeping your Blackberry software update is essential if you want to use the latest feature and keep it more secured. This was the second thing I checkout first after I had moved my contacts from my old phone to blackberry curve 8520.
All you need to do is go to this page and download a 31MB plugin after disconnecting your phone. Once downloaded connect your phone again and then run this software. It will check for any updates available and will update the Software as required. You wont be able to use your phone till the upgrade is done and do not disconnect it.
You will get clear options on what kind of updates will be made and you can add or remove applications which are part of your update package. Also It will make a backup your phone before upgrading. Just make sure you are doing it right.
You can also use the Black Berry Desktop Manager for keeping any installed applications update and Specific service providers update from here