To get a previous version of a file, login to Dropbox.com, browse to the file you'd like to get a previous version of, then hover over the file and click the blue arrow on the far right that'll appear beside the file change date. This menu lets you, among other things, restore old versions of the file. Select the "Previous Versions" button to see all of the versions of the file from the past 30 days on any free or paid account, or if you have the packrat upgrade enabled, then you can see unlimited previous versions. Hope that helps!