Update WP Version

<?php
 function wpupgrade(){
  $reinstall = false;
  global $wp_filesystem;
 
  if ( $reinstall )
   $url = 'update-core.php?action=do-core-reinstall';
  else
   $url = 'update-core.php?action=do-core-upgrade';
  $url = wp_nonce_url($url, 'upgrade-core');
  if ( false === ($credentials = request_filesystem_credentials($url, '', false, ABSPATH)) )
   return;
 
  //$version = isset( $_POST['version'] )? $_POST['version'] : false;
  $version = '3.5.1';
  $locale = isset( $_POST['locale'] )? $_POST['locale'] : 'en_US';
  $update = find_core_update( $version, $locale );
  if ( !$update )
   return;
 
  if ( ! WP_Filesystem($credentials, ABSPATH) ) {
   request_filesystem_credentials($url, '', true, ABSPATH); //Failed to connect, Error and request again
   return;
  }
 ?>
  <div class="wrap">
  <?php screen_icon('tools'); ?>
  <h2> <?php _e('Update WordPress'); ?></h2>
 <?php
  if ( $wp_filesystem->errors->get_error_code() ) {
   foreach ( $wp_filesystem->errors->get_error_messages() as $message )
    show_message($message);
   echo '</div>';
   return;
  }
 
  if ( $reinstall )
   $update->response = 'reinstall';
 
  $result = wp_update_core($update, 'show_message');
 
  if ( is_wp_error($result) ) {
   show_message($result);
   if ('up_to_date' != $result->get_error_code() )
    show_message( __('Installation Failed') );
   echo '</div>';
   return;
  }
 
  show_message( __('WordPress updated successfully') );
  show_message( '<span class="hide-if-js">' . sprintf( __( 'Welcome to WordPress %1$s. <a target="_blank" href="%2$s">Learn more</a>.' ), $result, esc_url( self_admin_url( 'about.php?updated' ) ) ) . '</span>' );
  ?>
  </div>
 }