Can you explain a bit more what you want to do please. In Reader, you cannot change the font in a PDF document, you would need Acrobat for that. You can change the font when you are adding text with the typewriter tool, or for other annotations, but it would be good to know exactly what you are trying to accomplish.
When you say it's "stuck" - does that mean that you can see a control to change the font, but the change you are trying to make does not get applied?
If it's the typewriter tool, you should see a properties toolbar with the option to change the font and font size. If you are talking about an annotation, you will have to change the font in the application Preferences in the Commenting section.