THe font setting for paragraphs and titles are global settings. You can change it globally by going to Theme--->change font. If you want to change font within a paragraph, that will be hard to do unless you know how to do html coding.
You can edit it like editing text in most word processing or text editing programs. Click on a text element, then select the text you want to change and start typing, or place the cursor and backspace to delete, etc.
