Why are you asking us? Not to be rude, but are you saying that your provider gave you a discounted key for an the current version of Office? And you want an older version? I don't think there is a downgrade path if that is what you are asking.
Can you please confirm whether its Office 2016 that they sold or it was Office 365 (Office 365 currently offers 2016 version of Office).
If they sold Office 2016, you would not be able to download Office 2013. However, there may be a work around if its Office 365. https://www.cloudappsportal.com/ https://www.cloudappsportal.com/