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It’s usually near the bottom-right corner of the screen in the same vicinity as the clock. Look for the blue and white open box icon. If you don’t see the icon, click the upward-pointing arrow to expand additional icons. A menu will appear.  It’s the second icon at the top of the window. This signs you out of Dropbox. A login screen will appear in case you want to sign in to another account. To re-link your Dropbox to Windows, click the icon, then enter your login information to sign in.

Summary:
Click the Dropbox icon in the system tray. Click the gear icon in Dropbox. Click Preferences…. Click Account. Click Unlink This Dropbox….