WordPress - Modify main theme shortcode through child theme. (eg. To modify "intro_btn_name" shortcode of main theme, use following code under child theme's functions.php)

functions.php

add_action( 'after_setup_theme', 'themewerx_child_theme_setup' );

function themewerx_child_theme_setup() {
    remove_shortcode( 'intro_btn_name' );
	add_shortcode( 'intro_btn_name', 'themewerx_intro_btn_name' );
}

function themewerx_intro_btn_name( $atts, $content = null ) {
	global $intro_btn;
	extract( shortcode_atts( array(
		'url' => '#'
		), $atts ) );
	$intro_btn = '<a class="button theme-button" href="'.esc_attr($url).'">'.$content.'</a>';
	return $intro_btn;
}