Installed (Parent) themes should never be changed because your changes will be lost after a theme update. Instead, use the child theme which is an extension of the parent theme and will not get overwritten when the parent theme gets updated.
This type of work is mainly for developers or someone who has basic knowledge of PHP and CSS. Please see the following pages for detail information on how to create and update your child theme:
- WordPress.org – Child Themes
- Elegantthemes – How to create a Child theme
- wpbeginner – How to Create a WordPress Child theme ( Video )
- managewp – How to Create a Child theme in WordPress
- wpmudev – How to Create a WordPress Child Theme
- Themify – Creating Child themes
Always remember to back up your website or make changes on the staging or test site before changing files.