The short answer is 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.
Here are the instructions for typing fractions in Microsoft Office 2013:
1. Place the cursor on the document where you want to enter the fraction, then click the Insert tab.
2. Under the Insert tab, find and click the Equation button.
3. Select Fraction under Equation Tools Designs option.
4. Select the fraction style you want and enter the numbers in the boxes.
