Microsoft today began offering a 64-bit version of Office for Mac to users on the Production channel. It was originally offered to Office Insiders back in July as version 15.24. Today's update is 15.25, which Insiders have had for a couple weeks.
Ever since Apple released Mac OS X 10.7 Lion back in 2010, the company's desktop OS has only supported 64-bit Intel processors; however, until Microsoft announced 64-bit Office, the productivity suite has only been offered in 32-bit. As Microsoft says, "unlike Windows, there is no choice between running a 32-bit or 64-bit version of the operating system."
Because of this, Microsoft will not continue to offer a 32-bit version of Office for Mac. Once each channel has transitioned to 64-bit, that's all that will be available.
Some customers might not be prepared for a switch to 64-bit Office; therefore, the company will be offering one more 32-bit build. Version 15.25 is available in 32-bit, but you'll have to jump through a couple of hoops to get it. Microsoft provides instructions for installing the update here, whether you've already installed the 64-bit update or not.
If you're running Office 2016 for Mac, you'll see an update waiting for you.