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Symptoms
You can't change an equation in a document.
Resolution
You may be trying to change an equation that was created by using an addin in a version of Microsoft Office Word prior to Microsoft Office Word 2007. To change these equations, you need to use the addin that was used to create the original equation: Microsoft Equation 3.0 or Math Type.
Equation 3.0 was included in previous versions of Word and is available in Office Word 2007. Math Type was not included in previous versions of Word but was available for purchase. If you purchased Math Type, you need to have Math Type installed.
Use Equation 3.0 to change an equation

Doubleclick the equation that you want to change.

Make the changes that you want.
To get help in Equation Editor, click Equation Editor Help Topics on the Help menu.
Use Math Type to change an equation
To change an equation written with Math Type, you need to have Math Type installed.

Doubleclick the equation that you want to change.

In Math Type, change the equation.

On the File menu, click Close and Return to name of the document that contains the equation.
To get help in Math Type, click Contents and Index on the Help menu.
Use Office Word 2007 features
To change an equation by using features that are built into Office Word 2007, you need to convert your document and save it as an Office Word 2007 file. However, if you convert your document, you will not be able to use previous versions of Word to change any equations in the document.

Click the Microsoft Office Button , and then click Convert.

Click the Microsoft Office Button , and then click Save.
Note: After you convert your document, you need to save the document as an Office Word 2007 file, or it will revert to the previous version.