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<?php
  $node = node_load(1); // 1 is node id we want to load

  // Change field value 
  $node->field_product_description[$node->language][0]['value'] = 'This is a very nice and useful product. Buy it now! We want your money!';

  // Get the numeric id of your field by passing field name
  $info = field_info_field('field_product_description');
  $fields = array($info['id']);

  // Execute the storage function
  field_sql_storage_field_storage_write('node', $node, 'update', $fields);

  // Clear field cache
  cache_clear_all("field:node:$node->nid", 'cache_field');