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When you open a document containing an equation that was written in a version of Word prior to Word 2007, you need to use Equation 3.0 or the addin that was used to write the equation to change it. Equation 3.0 is still available in Word 2013 and when you click in an equation written with Equation 3.0, it will open automatically.

Doubleclick the equation that you want to edit.

Make the changes that you want.
If you convert an older document to the current format, you can use Word 2013’s more powerful builtin tools to write an equation. However, any old equations written using Equation 3.0 or using an addin must be manually recreated if you wish to use Word 2013’s builtin equation editor.
To convert a document, click File > Info > Convert.