If you use the maths equations in Insert > Equation you get font which is in Cambria Math (sic) font which can’t be changed in the usual way. Running this macro over it will change it to Arial 11. It will also change the font of any other equation in the document. If you change any of the characters in the document it will revert to Cambria Math (sic) font so run the macro over it again after you have finished.
Sub ChangeCambriaMath() For Each equation In ActiveDocument.OMaths equation.Range.Font.Name = "Arial" Next equation End Sub