Change the default message

Use the wpmem_restricted_msg filter:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_restricted_msg/

I paste the php code that u provided from the url but display the content that I want and after that the default text.

I would like to convert the default text with the new one that I create.

You can do that using the wpmem_restricted_msg filter I directed you to:

add_filter( 'wpmem_restricted_msg', 'my_restricted_msg' );
 
function my_restricted_msg( $str ) {

    $str = '<p>Your custom post restricted text.</p>';
 
    return $str;
}