Hi Bill, to my knowledge Office 365/2016 has no 'reset' option. Your best bet would be to rename the global template file forcing Word to re-create the default version.
I don't know if this is actually going to help as it resets the global template not actual settings if I am correct. It would seem that to actually restore all settings to default takes a registry hack per:
https://www.youtube.com/watch?v=1zBMp1BNpBsPlease note that I have not tried this hack!To reset the global template.
1) Exit any open Office apps.
2) Open File Explorer and set the view options to allow viewing of hidden files/folders. You MAY also need to set to show protected files/folders but I don't think that will be the case.
3)Navigate to “C:\Users(your username)\AppData\ Roaming\Microsoft\Templates.” and rename "Normal.dot" to whatever you want.
4) Open Word and it should re-cre4ate the default "Normal.dot" template file.
This method can actually come in handy. Say that you rename Normal.dot to Publish.dot. In the future, if you need to publish another book, just rename Publish.dot back to Normal.dot and your template is restored.