Microsoft removed Equation Editor features in Office for security reasons in January 2018 patches, which will make existing equation objects made with the tool read-only.
Per Microsoft recommendations here: <https://support.microsoft.com
/en-us/help/4057882/error-when-editing-an-equation-in-office>, the resolution is:
You can insert and edit new equations using the editor built into Office version 2007 or later. For more information about inserting and editing equations, see Write, insert, or change an equation. However, you will not be able to edit existing equations that were inserted using Equation Editor 3.0 in that manner.
Alternatively, the third-party app MathType enables you to edit Equation Editor 3.0 equations without security issues. MathType is now part of the Wiris Suite. You can download a free MathType 30-day trial at: <http://www.wiris.com/en/equation_editor/microsoft>
If you want to obtain a long-term copy of Mathtype, an academic license is available at $57 per person. Purchase details are available at <http://store.dessci.com/
?>. CIS' software team has checked with the vendor, and an extra 10% discount is available if a purchase of greater than 50 copies is made. At this time, we don't expect demand to be high enough to reach this threshold, but if you're aware of a large number of people who want to pursue this discount, please let us know and we can attempt to aggregate a group order. Otherwise you are free to purchase directly.