Add shortcode with open close function

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>';
    }