WordPress is a commonly used blogging system with thousands of different themes (designs) that all have their own unique ways of handling fonts.
Because of this, there are no one way to change the font guaranteed to work for your specific theme.
Unless you can find a specific font setting in your theme options, you will have to know some basic CSS and HTML rules in order to change the font yourself.
Try looking for a WYSIWYG way to edit your WordPress font by going to your control panel, and then:
Appearance -> Theme settings
Appearance -> Customizer
If you have a theme which has not made it possible to change easily, you will have to look in your source code and find CSS selectors that apply to the various font elements you wish to change.