To change the functionality of a shortcode you must first remove_shortcode( 'shortcode_name' );
where shortcode name is the name of the shortcode. Add the shortcode back with your NEW function.
A simple example to follow what you might be needing:
remove_shortcode( 'mybutton' );
add_shortcode( 'mybutton', 'my_shortcode_function' );
my_shortcode_function( $atts, $content = "" ) {
return '<a href="http://example.com">' . $content . '</a>';
}