From this articel from 2013
The 32-bit version of Office is installed by default. Even if your
computer is running a 64-bit version of Windows, we recommend that you
install the 32-bit version of Office, because it is more compatible
with the applications and add-ins that work with Office. If you do
want to install the 64-bit version of Office, you can do so by taking
the following steps: Office 365 Home Premium Preview: Go to
www.office.com/myaccount, click Languages and install options, and
then click Additional install options. Other Office 365 plans: Log
into your account at www.office365.com and choose the option to
install the 64-bit version of Office. It is not possible to install
both the 64-bit version and 32-bit versions of Office on the same
computer.