If you still have the valid License Key, you can download Genuine MS Office 2013 from the Microsoft website.
Before installation, you will need to log into your MS/Live account (or create a new account, if you don't yet have one), then enter your valid license key, and the installation will commence.
Without a license ... you'll have to purchase it again.
Fortunately Office 13 and 16 are cheap these days.
Cheers,
Gaius