1) Download child theme from the Downloads area https://www.8theme.com/downloads
2) Extract it and change the folder name to xstore-niche-child for example. Also you can edit the child theme style.css insde and change the child theme title, for example http://prntscr.com/ti6zut
3) Upload the child theme folder to wp-content/themes/
4) Go to Dashboard > Network admin > Themes > click on Network activate for your child theme.
5) Go to your site dashboard and activate this child theme.