Learn@UW - Equation Created by Equation Editor Cannot Be Edited
If you created an equation using the Graphical Equation editor in a document using the HTML editor, you should be able to edit the equation when later editing the document by placing your cursor over the equation editor box (Sigma in a yellow box) and then clicking the 'Graphical Equation' (Sigma) icon.
If you created an equation using the Graphical Equation editor in a document, you may not be able to edit the equation.
This apparently works for Content topics but not for News items, Discussion posts, Glossary items, and possibly other non-Content tools that use a slightly different HTML editor than the Content tool.
In tools other than Content, you see just a single small square where the equation is expected to be in the 'Edit Equation' interface. Then, you have to Cancel out to avoid losing the original equation.
This is a known issue.