The margins are set through Page Setup. To change the margins for all new documents, set the desired margins in the dialog and click Set as Default. (See http://wordfaqs.ssbarnhill.com/CustomizeNormalTemplate.htm for more.)
For duplexing, you will need to set this as the default through Windows Settings | Devices | Printers and Scanners. Select your printer and click on Manage. On the Manage your device page, click on Printer properties, and then choose the desired settings in the Properties dialog for your printer.