If you are in the new email format, it's very easy. Open the address book, then select Address book again in the left pane. Click on the "hamburger" menu to the right and you will see Export. Click on that and it will automatically export your contacts as a .csv file to your default download folder on your computer. If you are still in the old format, it's more complicated:
Open the address book from the Inbox page, then on the landing page click on "Export all". On the next box, it should be set to Microsoft Outlook by default. Click on Export again.