PHP files inside include directory not getting modified

Hello I am using a child theme to modify the WPLMS theme. Normally, if a file inside a directory in the main theme has to be overridden by the child theme, a copy of that file should be kept inside a directory of the same name (maintain the same file structure) and then modified. When I copied the register.php file inside a directory named 'includes' in the child theme root folder, my changes are still not reflecting on the website. I am having to edit the main wplms register.php directly (which will probably get overridden when you update the theme). Please tell me if I am making a mistake somewhere and if I should try some other way. Thanks.