Hi there, If you have not yet done so, I would suggest creating a child theme so that any customizations you make will not be overwritten by a future theme update. Alternately you can use the Edit CSS function in the Jetpack plugin, or use one of the other Custom CSS plugins that will keep CSS changes from being overwritten.
Jetpack plugin
Child Themes
Child Theme creation plugins
Custom CSS plugins
There is usually a general font-family declared in a body selector in the CSS, but some themes may set different fonts for different things, like headings (h1-h6) or widget, post and page titles, so we would need to know which text in your site you are wanting to change. Also, what font-family are you trying to change things too?
A link to your site, if it is live, would be quite helpful, and also, if you could paste the exact CSS you are trying to use, that would be helpful too. You can paste the CSS here between backticks or between code tags such as this:
<code>your_css_here</code>
