Here is a link to the Microsoft website for a solution.
The short answer is, for older versions of Word (2007, 2010, 2013), you need to buy a new font. Out of the box, Office 2010 and 2013 have only Cambria with all of the math characters that are needed for the Equation Editor: "If you're really serious about changing the font, you may have to purchase an add-on from Microsoft called Kutool."
Click on the Equation Editor for more information about changing fonts for equations one-by-one as well as how Kutool will help you. This tool will work in Word 2003/2007/2010/2013/2016.
In Word 2016 you can change font sizes, styles or even paragraph style in every equation like a usual text.
To change a font size & style in an equation (how to add an equation, see Working with Microsoft Equation), do the following:
1. Select an equation or part of it which you want to change.
2. Right-click in it and choose Font... in the popup menu (or click Ctrl+Shift+F):
3. Change the settings like a usual text in Word 2016.
To change a paragraph style, select an equation or part of it which you want to change, right-click in it and choose Paragraph... in the popup menu. Then change the settings like a usual paragraph in Word 2016.
A Walsh visitor suggested an alternative fix (thanks to Jorn v D):
Select the text; go to "Design" and click "abc Text" on the left. Afterwards you can change it to whatever you like.
