How to prevent XElement from decoding character entity references
string originalXML = @"<Description><data>Mark's Data</data></Description>"; //for illustration purposes only
string encodedApostrophe = originalXML.Replace("'", "&#39;");
XElement xe = XElement.Parse(encodedApostrophe);

originalXML.Replace("'", "&#39;")

XElement xe = XElement.Parse(originalXML);

var data = xe.Element("data");

data.Value = data.Value.Replace("'", "&#39;");