An update is rolling out ot existing Microsoft Office for Mac users which transitions the suite of applications from 32-bit to 64-bit. This is expected to boost performance and allow the implementation of new features.
This means that Word, Excel, PowerPoint, Outlook, and OneNote get upgraded to 64-bit for anyone running Retail, Office 365 Consumer, Office 365 Commercial, and Volume License installations of the software. What's more, the August release (version 15.25) will arrive just like any other update to the apps.
Source: Engadget