You are updating the theme in a proper way but if you have modified the coding inside the theme then it will messup the theme. You need to use child theme if you wish to customize the theme.
A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme. Child theme allows you to modify, or add to the functionality of that parent theme. A child theme is the safest and easiest way to modify an existing theme, whether you want to make a few tiny changes or extensive changes. Instead of modifying the theme files directly, you can use a child theme and override within.
