There is a general move toward 64-bit software these days, but if you have an Office 365 subscription, you might think there's only a 32-bit version available. In fact, there is a 64-bit version if you look in the right place.
There are various advantages over 32-bit, such as the ability to work with larger files. To grab a 64-bit version of Office 365 — providing you have paid for a subscription — pay a visit to the Office website and click the Install Office button. Sign into your account then click the Language and install options link in the Install Information section.
Look to the bottom of the installation window and click the Additional install options link. Click the Version menu, select the Office (64-bit) option, and then click Install.
As How-To Geek points out, there is potential for compatibility issues with some plugins, but if you want to stick with 64-bit software, you can.