I never entered my Product Key for Office 2013, how can I enter it now?
Please try the following:
Open any of the Office applications, click file, and then account. You should find, Change Product Key in the right hand activation section of this page.If this is not displayed you can also change the product key with the following method.
To change your product key go to Control Panel >> Programs and Features, then right click on Microsoft Office 2013 and select Change. You will then have the option to enter a product key.
Applies to Microsoft Office 2013 Student personal use license prior to Office 365.