Hello,
You need to install and activate child theme (follow documentation https://xstore.helpscoutdocs.com/article/32-child-theme ) and then if you need to do any changes (styles or code) use child theme. How child theme works you can read here https://codex.wordpress.org/Child_Themes
So, add your functions into child theme functions.php instead of the parent theme. All the changes made in parent theme will be lost after theme update.
Let me know if you need any other detailed explanation.
Regards
