Click on File (Located in the top left of outlook)
Click on “Open & Export” from the left hand menu, then
Click on “Import/Export”
Choose “Export to a file” and click Next
Choose “Outlook Data File” and click Next
Minimize the folders by clicking on the arrow located to the left of the name, then select the folder you wish to backup.
Choose a location for the backup to be saved, or leave as the default.
Make sure “Do not export duplicate items” is selected and click finish
