Open Word
Go to the equation you want to edit
Click inside the equation
Select the equation text
Go to the Home tab
Choose the font you want from the Font box
If the font does not change, open the Equation Tools Design tab
Click the small dialog launcher in the Font group if available
Set the desired font for equation text
Save the document
To change the default for new equations, create a new equation and set the font before inserting more equations
If needed, use the Normal template or a style to keep the font consistent