Hello,
Yes, if you will do the changes/customization in Parent Theme files then surely it will override when you will update the Theme in future, So, you need to change/customize it using Child Theme.
Here are basic points about child theme usage with our theme:
1) If you need to customize any theme files or add a lot of CSS changes to your web-site you need to use child theme.
2) Here is the link that demonstrate how it works: https://developer.wordpress.org/themes/advanced-topics/child-themes/
3) You don’t need to create child theme from scratch because we already made it for you (you have found in in theme package downloaded from themeforest)
4) Now copy the same file 404.php from Parent Theme to Child Theme under same folder/same path and Customize it as per your requirements.
5) When you are done with child theme set up, you can freely update PARENT theme without loosing any configurations or changes made in child theme files.
Best Regards.
.
