What is the best way to override a plugin's function from child theme ?

I have a plugin called abcd-plugin, which contains two files:

1.	abcd-single-portfolio-item.php
2.	single-portfolio.php

The function is defined in abcd-single-portfolio-item.php and used in single-portfolio.php.

How can I override this plugin’s functionality from the functions.php file of my child theme?

When I directly copy and modify the code in my child theme, it doesn’t have any effect.

Found a solution that worked for me

/**
 * Remove the portfolio template from the parent theme/plugin functions.
 */
add_action( 'init', 'remove_abcd_item_template', 20 );

function remove_greennature_portfolio_template() {
    // Remove the filter that modifies the single template.
    remove_filter( 'single_template', 'remove_abcd_item_template' );
}

had to copy the single item template in child theme root directory, replaced the method with my own custom method