Not quite sure where you are seeing a brown bar as it is black for me. So after the upgrade you don't have the black bar at the top of the page when in the backend?
If you are missing the black bar I would suggest deactivating all plugins and switching to the default Twentyten/Twentyeleven themes and seeing if the issue still persists. If it does still persist it could be an issue with a corruped/missing file during the upgrade in which you could try reinstalling through Dashboard->Updates.
If that doesn't work I would next try a manual reinstall to make sure all files are uploaded.
If however you do have the black bar up at the top you can log out by hovering over the far right area where it displays 'Howdy, [username]' along with your avatar and a menu will popup allowing you to log out.