Before anything else you'll first need to update your Office software. The build you cited dates back to August of 2015. Your version of Microsoft AutoUpdate [MAU] must be equally obsolete so it is unable to recognize the updates that are available.
Further, v15.13.3 is an obsolete Volume License release [for businesses] so you may need to contact your IT department to update it for you if you're still eligible.
If your License still is in effect & you have the option to update on your own, you can download the MAU update by clicking this link. After installing it Restart your Mac then launch one of the Office programs. You shortly should be prompted by MAU to update to the current release for your License type... If not, go to Help> Check for Updates to launch MAU in order to retrieve the Office update.
Once the software is up-to-date you'll have the option to customize the Quick Access Toolbar [QAT] by going to Word> Preferences > Ribbon & Toolbar - Quick Access Toolbar but I don't believe the Volume License for Mac Office 2016 provides the option to customize the Ribbon.