Installed (Parent) themes should never be changed because after theme update your changes will be lost. Instead use child theme which is an extension of the parent theme and will not get overwritten when parent theme gets updated.

Resources

This type of work is mainly for developers or someone who has some basic knowledge of PHP and CSS. Please see the following pages for detail information on how to create and update your child theme:

• WordPress Codex
• Elegant themes Child theme tutorial
• WP Beginner Child theme Video tutorial
• ManageWP Creating a Child theme

Always remember to backup your website or make changes on staging / test site before changing files.