If you want to have the latest stable version of Dropbox, you can always get it from our download page.
To tell which version of Dropbox you're using, simply hover your mouse over the Dropbox icon in your system tray. A small status box will appear with the version number. You can also find the version number by visiting your Dropbox preferences and selecting the Account tab. The Dropbox version number appears in gray.
Auto-upgrades are rolled out gradually after a new update is available. We're very conservative about auto-updating. We never want to risk breaking a working version of Dropbox.
We rely on enthusiastic people in our forums to try the latest versions of Dropbox and send us feedback. This helps us to work out all the kinks before we update everyone else. If you like to be on the cutting edge, strap on a helmet and visit the forums for the very latest in-development version of Dropbox. You'll see a post for it at the top of the main page and the "Everything Else" category.