Hello,
Please read about the child theme
https://codex.wordpress.org/Child_Themes
When you edit some files in the parent theme and don’t want to lose modifications after theme update you should copy the edited file to the same file structure in the child theme.
Also if you use child theme you need make style changes in style.css of the child theme, you should write only the code for the element you want to modify and don’t copy whole parent style.css.
Besides we always recommend to create backup before each update and modify theme files in the child theme.
Regards,
Rose Tyler.