If you have a Verizon BlackBerry Curve 8330 or BlackBerry Pearl 8130 you can now download an update of OS 4.5 from the Verizon official website.
Unfortunately if you own a BlackBerry 8830 there is no update OS 4.5 available for you yet, but if you want to be kept up-to-date with the latest information and download for your BlackBerry then on the CrackBerry Forum you will see all the reviews and news on the latest OS 4.5.
If you have a BlackBerry and what to learn more about updating software for it, then check you can check out this article from crackberry.com where you can find all you need to know about your BlackBerry.