Workbooks can be downloaded using the Download Workbook link in the upper right corner of the view. The downloaded workbook can be opened with a version of Tableau Desktop. Downloading the workbook from the server is the same as selecting Server > Open Workbook in the desktop application. The workbook can only be opened if it is still published to the server.
This option is only available if you've been given the Download File permission by the author of the workbook or an administrator. Refer to Setting Permissions to learn more.