Microsoft released today its first ever 64bit version of Office for Mac.
The version, numbered as 15.25.0, also includes several additional improvements and bug fixes that are detailed in the link below:
https://support.microsoft.com/en-us/kb/3179163
As described in the notes, be aware that you might encounter issues with a small number of add-ins.

To install the Microsoft Office for Mac 64bit update just start AutoUpdate by selecting the -Help- menu and -Check for Update- within any of its applications.