Create a child theme in wordpress

The project is to create a child theme to change 2 files ([login to view URL] and [login to view URL]). Hi have this badge on top left of every page. You can see it in action here [login to view URL] This is done this way: 1. [login to view URL], line 97, added code "<a href="/" id="badge"> </a>" 2. css/[login to view URL], line 3-4, added code "a#badge { position: absolute; right: 100px; top: 80px; z-index: 9999999999; background: url(../img/[login to view URL]) no-repeat; width: 143px; height: 142px; line-height: 0; display: block; } a#badge:hover { background-image: url(../img/[login to view URL]); }" The theme is [login to view URL] Everytime i update the theme i have to edit this 2 files. Project is to create a child theme, so i don't have to edit files everytime.