Where is Equation Editor?
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Microsoft Word 2010 includes builtin support for writing and changing equations. As a result, Equation Editor is not used, but instead equations are edited directly from within Word. To do this, click the Insert tab then click the Equation button. For information about writing equations in Word 2010, see Write, insert, or change an equation.
If you wrote an equation using Microsoft Equation 3.0 in a previous version of Word, however, you still need to use Equation 3.0 to change that equation.
Change an equation that was created with Equation 3.0

Doubleclick the equation that you want to edit.

Make the changes that you want.
To get help in Equation Editor, click Equation Editor Help Topics on the Help menu.
Use the Office Word 2010 features
When you open a document that was written in a previous version of Word, you cannot use the enhanced equation editor functions for writing and changing equations unless you convert your document to the new format. To convert your document, do the following:

Click the File tab, and then click Convert.

If you are prompted, click OK.

Click the File tab, and then click Save.
Note If you convert and save your document as a Word 2010 file and then add equations to it, you will not be able to use previous versions of Word to change any of the new equations.