ios5 update "warning"

When i hit update on itunes on my mac... i get this mssg:

"Updating to iOS 5.0 will delete all of the apps and media, including iTunes Store purchases, on your iPhone. To preserve your content, apply this update on the computer where you sync apps, music, videos, and photos. Updating on this computer will only preserve contacts, calendars, text messages, and other settings. Please do not interrupt the update, which may take an hour or longer to complete. iTunes will verify the software update with Apple".

Im on the computer i do this. Any help will be apreciated.... thnx!