Use the Theme Customizer to edit your WordPress theme
You can access it from Appearance > Customize or from Appearance > Themes > Customize button in the active theme's thumbnail. This loads the Customize page from which you can apply changes to your WordPress theme and see the result immediately.
In most cases you can solve issues with the theme customizer by disabling your plugins in the WordPress dashboard. You could try deactivating your plugins one by one to identify the plugin that is causing the issue and then just replace the specific plugin with a plugin that doesn't cause issues.