Child Theme
When making alterations to WordPress it is good practice to create what is called a child theme. This is a theme based on an existing theme, which allows you to add to the main theme files without actually changing the original theme. This is important because when you update a theme it overwrites any changes you made to it, which means you would be required to make all your changes again every time you update your theme.
Creating a Child Theme
This video covers how to simply create a child theme by installing and using a child theme plugin.