How To Downgrade from 365 Office 2016 to Office 2013.

1. Use the Fix it tool to remove the current Office 2016. 

Uninstall Office 2013, Office 2016, or Office 365 from a Windows computer

2. Install Office 2013 via the URLs below:

For Office 365 Business or Office 365 Business Premium:

32-bit, English:

https://c2rsetup.officeapps.live.com/c2r/download.aspx?productReleaseID=O365BusinessRetail&platform=X86&language=en-us&version=O15GA&source=O15OLSO365

64-bit, English:

https://c2rsetup.officeapps.live.com/c2r/download.aspx?productReleaseID=O365BusinessRetail&platform=X64&language=en-us&version=O15GA&source=O15OLSO365

You will need to do this as well in order to prevent your computer from upgrading to 2016

https://support.microsoft.com/en-us/kb/3097292