Can you be more specific about what you are trying to do.
Are you trying to change the font in a style?
If you select a Style that is in use (Ribbon - Home - Styles - click expand arrow which shows a list of styles in use).
Each style in the styles pane shows a drop-down arrow when you mouse over it. Click the drop-down arrow and select Modify.
From the dialog box that opens you can select a new font for that style.
Select Font from drop-down list and click OK