The easiest way to customize your WordPress theme is using the WordPress Customizer. You access this in one of two ways: When viewing your site (when you are logged in), click the Customize link in the admin bar at the top of the screen. In the admin screens, click on Appearance > Customize.
Click on this to open the default theme's settings page. Here, you'll find a list of all the WordPress themes installed on your site. Click on the “Reset Theme” button next to the theme you want to reset to its default settings. Once the resetting process is complete, your theme will be back to its default settings.