Fonts keep changing in Word 2013, and setting the one I want as default does nothing.

http://wordribbon.tips.net/T006072_Finding_Where_Templates_Are_Stored.html - Ribbon

http://word.tips.net/T001668_Opening_a_Template.html

http://addbalance.com/word/normaltemplate.htm 

What do you mean fonts keep changing?

If using the default button in the Font dialog does not work, you have 2 more options.

Click on the dialog launcher (it is the dirty squiggle on the right end of the Styles group name bard in the ribbon) button in Home Tab > Styles group to show the Styles Pane. Right click on the NORMAL style and select modify.  Make sure the "New Docuemnts based on this template" radio button at the bottom of the dialog is also turned on.  Close all word sessions, including Outlook, to save the change to your Normal template.

If that does not work, then you will have to open the normal template itself for editing, making the change to the normal style there.

Find Normal.DOT / Find Normal.DOTM

http://wordribbon.tips.net/T006072_Finding_Where_Templates_Are_Stored.html - Ribbon

http://word.tips.net/T001668_Opening_a_Template.html

http://addbalance.com/word/normaltemplate.htm 

The links above show a few different ways of finding normal. Here is one more:

In Word, press Alt+F11; the Visual Basic Editor opens. Press Ctrl+G to activate the Immediate window. In that window, type (or paste)

     Application.NormalTemplate.OpenAsDocument

and press Enter. Normal opens, but note that it may not be on top of all windows.

I have the same problem. Fonts and fonts sizes change randomly when I change parts of my document. I even had a section that stuck on a heading style. Even when I changed it back to normal it would instantly change back to Heading...... Word 2013 is a heap of ****

If the changes are completely random, I would suspect a poorly designed add-in.

What are you doing, specifically, when the changes occur? Are you just typing text?