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Open the U-M Dropbox website. A quick link is available via the Applications tab under Other for your convenience:

Sign in with your uniqname and UMICH password, and click on the Upload button on the Dropbox Home page:

Browse to /afshome/uniqname to see your AFS files (click the < arrow and then the hard drive icon to see it):

Finally, select the files you wish to archive in Dropbox.