The default font is the font associated with the normal paragraph style in the normal template (document templates will have their own defaults). If you create documents from appropriate templates the default font is somewhat irrelevant, but if the font setting is reluctant to stick, open the normal template for editing in Word - Offioce Button > Open > Select Normal.dotm. If you have not changed the default location enter or copy
%appdata%\Microsoft\Templates
to the address window of the Windows Explorer file window and you will be taken to the folder that contains the default template. Note that this folder is normally hidden.
See which style is highlighted in the quick styles list. That is your default style:
Right click the highlighted style in the quick styles box and select 'Modify'. Make whatever changes you require.
Type some text in the document area. Make sure the style you have just modified is applied to it by clicking the highlighted style. Delete the text and save and close the template.
Create a new document. Type some text. Is the font what you set?