how to programmatically access the builtin properties of an open xml worddoc file

An explanation of the following method can be found here, but pretty much you need to pass in the properties that you want to get out of the core.xml file to this method and it will return the value:

public static string WDRetrieveCoreProperty(string docName, string propertyName)
{
   // Given a document name and a core property, retrieve the value of the property.
   // Note that because this code uses the SelectSingleNode method, 
   // the search is case sensitive. That is, looking for "Author" is not 
   // the same as looking for "author".

   const string corePropertiesSchema = "http://schemas.openxmlformats.org/package/2006/metadata/core-properties";
   const string dcPropertiesSchema = "http://purl.org/dc/elements/1.1/";
   const string dcTermsPropertiesSchema = "http://purl.org/dc/terms/";

   string propertyValue = string.Empty;

   using (WordprocessingDocument wdPackage = WordprocessingDocument.Open(docName, true))
   {
      // Get the core properties part (core.xml).
      CoreFilePropertiesPart corePropertiesPart = wdPackage.CoreFilePropertiesPart;

      // Manage namespaces to perform XML XPath queries.
      NameTable nt = new NameTable();
      XmlNamespaceManager nsManager = new XmlNamespaceManager(nt);
      nsManager.AddNamespace("cp", corePropertiesSchema);
      nsManager.AddNamespace("dc", dcPropertiesSchema);
      nsManager.AddNamespace("dcterms", dcTermsPropertiesSchema);

      // Get the properties from the package.
      XmlDocument xdoc = new XmlDocument(nt);

      // Load the XML in the part into an XmlDocument instance.
      xdoc.Load(corePropertiesPart.GetStream());

      string searchString = string.Format("//cp:coreProperties/{0}", propertyName);

      XmlNode xNode = xdoc.SelectSingleNode(searchString, nsManager);
      if (!(xNode == null))
      {
         propertyValue = xNode.InnerText;
      }
   }

   return propertyValue;
}

You can also use the packaging API:

using System.IO.Packaging.Package;

[...]

using (var package = Package.Open(path))
{
    package.PackageProperties.Creator = Environment.UserName;
    package.PackageProperties.LastModifiedBy = Environment.UserName;
}

That works also for other open XML formats like power point.