You don’t need to install anything to write or edit math equations in Microsoft Word 2013. Just choose Insert > Equation.
Equation Editor (Microsoft Equation 3.0) was included in previous versions of Word and is still available in case you need to edit an equation written in a version of Word older than 2007. Doubleclick the equation you want to edit, and the equation editor will open automatically.
Important: This feature isn’t available in Office on a Windows RT PC. Want to see what version of Office you're using?
If your document is older than Word 2007, convert it to the newer format to use the new enhanced editor:

Choose File > Convert.

If a popup window appears, click OK.

Choose File > Save.
Note: If you convert your document to the new format and then add equations to it, you can’t use previous versions of Word to change the new equations.