Customizing contextual help for every page

You can add this code in your functions.php file.

function example_contextual_help( $contextual_help, $screen_id, $screen ) {
    //echo 'Screen ID = '.$screen_id.'<br />';
    switch( $screen_id ) {
        case 'my_plugin_page_1' :
            $contextual_help .= '<p>';
            $contextual_help = __( 'Your text here.' );
            $contextual_help .= '</p>';
            break;
        case 'my_plugin_page_2' :
            $contextual_help .= '<p>';
            $contextual_help = __( 'Your text here.' );
            $contextual_help .= '</p>';
            break;
    }
    return $contextual_help;
}
add_filter('contextual_help', 'example_contextual_help', 10, 3);

To get the screen_id of the particular page, just uncomment the first line in the above function and check your plugin page’s contextual help. Then add the screen id to the switch case like ‘my_plugin_page_1’. Incase your plugin is a single page plugin (or few pages) then you could just do a if condition (any one would do) instead of the switch cases in the following manner.

if ( $screen_id == 'my_plugin_page' ) {
   $contextual_help .= '<p>';
   $contextual_help = __( 'Your text here.' );
   $contextual_help .= '</p>';
}
return $contextual_help;