This procedure is provided for compatibility with earlier versions of Word. Use this procedure to edit an equation or expression that you want to share with earlier versions of Word. If you do not need to share the file with earlier versions of Word, use the Document Elements tab to open and edit mathematical equations or expressions that were created in earlier versions of Word or MathType. For more information, see Insert or edit an equation or expression.

Doubleclick the equation.

Use the Equation Editor tools and menus to edit your equation.
Tip: For assistance editing an equation, click the Equation Editor Help menu, and then click Equation Editor Help.

To update your document with the edited equation, on the Equation Editor menu, click Quit Equation Editor.
See also
Insert an equation in a document to open in an earlier version of Word