The first thing you’ll need to know for this is if the theme you’re currently using supports custom typography.
If it does, you’ll see the available options under Appearance → Theme Options in your blog’s dashboard.
Don’t forget you can quickly search for themes with custom typography using the feature filter in the Themes section.
Using the visual editor
If your theme does not support custom typography, you’ll be able to change the font-family or font size using the visual editor, when editing your posts.
To do that, use the following buttons in your visual editor:
If it doesn’t, try using a custom header image.