A family of Microsoft word processing software products for creating web, email, and print documents.
There is no single place to set the defaults for all Office applications. My instructions below apply to Word for Windows, the desktop version.
In Word for Windows, you can change the default font through the Font dialog box (Ctrl+D).
Make the desired changes and then click Set as Default. When prompted, choose "All documents based on the <template name> template" option.
Later, if/when you are prompted to save your template, choose to do so.
The modification is saved to the template whose name is indicated in the message (usually Normal.dotm unless you specifically based your document on another template) and applies to the current document and to future documents based on the template. Existing documents that you want to modify must be dealt with one by one.
Even after you have changed the default, individual styles may be set to override the document default font.