If I made all changes to Child them then I have to use Child theme for my website theme. So why to update then main theme?
Read this first.
A child theme inherits its functionality from parent theme. So child themes is a way to build on top of whatever already exists within the parent theme. This allow you to upgrade parent theme (bug fixes, security updates, new features) without loosing any modifications.
There is no need to be two themes "active" in the same time. When child theme is active, all parent theme functionality works in the background.
For example if you have made changes to parent theme
header.php, you should copy that file to child theme. When there is
header.php in child theme and in parent theme, child theme
header.php takes over.
Normally updates are for parent theme
functions.php and some added styles and template files.