How can I add/update post meta in a admin menu page?

You will need to use an Ajax function. Which requires javascript to send the form data to your php Ajax function which you can use to run update_post_meta();

Example:

Form Html:

<form>
<input id="meta" type ="text" name="2344" value="<?php echo esc_html( get_post_meta( 2344, '_your_key', true) ); ?>" />
</form>

Javascript:

jQuery(document).ready(function() {
    jQuery("form").submit(function() {
    var post_meta = jQuery("input name=[2344]").val();
    var ID = jQuery(this).attr("name");

    jQuery.ajax({
           type: "POST",
           url: ajaxurl,
           data: {
           action: "update_meta",
           post_id: ID,
           meta: post_meta,
          },
        success: function( data ) {
          //do something
     }
   });
 return false;

});

PHP function:

add_action( 'wp_ajax_update_meta', 'my_function' );
function my_function() {
   $post_id = $_POST['post_id'];
   $meta = $_POST['post_meta'];

  update_post_meta( $post_id, '_your_key', $meta );

  echo 'Meta Updated';
  die();
}