public string ToXML(object oObject)
    {
        XmlDocument xmlDoc = new XmlDocument();
        XmlSerializer xmlSerializer = new XmlSerializer(oObject.GetType());
        using (MemoryStream xmlStream = new MemoryStream())
        {
            xmlSerializer.Serialize(xmlStream, oObject);
            xmlStream.Position = 0;
            xmlDoc.Load(xmlStream);
            return xmlDoc.InnerXml;
        }
    }

<fProjectGID><fProjectGID/>

XmlDocument doc = new XmlDocument();
doc.Load(file);
XmlNode node = doc["Root"]["fProjectGID"]; \ Go to your desired Node
node.RemoveAll(); \It Will remove the  xsi:nil="true"
node.InnerText = " Put Here your desired text";  \ Any Inner Text You want.