There are two main ways to change fonts in your theme:
1. → allows you to select from a small set of fonts and change global sizes for the element selected (general, titles etc). You can easily reset your changes by clicking Clear on an individual element’s tab, or at the top to reset all settings back to default.
See your theme’s Typography Manager documentation for details
2. Import a custom font and then define it for the element(s) you want using Custom CSS. Plugins like WP Google Fonts or the WordPress Typekit plugin help make this a lot easier by providing a font selector, and some basic element checkboxes. You can choose to use the plugin’s custom css fields, or the one in Theme Options OR the one in the post itself if you want to make that post unique. To go back to the default, you simply remove or comment out the styles.
What you should not do is edit the theme’s stylesheet.