Try with below code:
string xmlData = @"<project>
<projectdetail id="1">
<Title>Dominic Crossroad</Title>
<platform>Android</platform>
<ggi>1522</ggi>
<dbname>dominicroad</dbname>
<gversion>1.0.0, 1.2.0i, 2.0.0j</gversion>
</projectdetail>
<projectdetail id="2">
<Title>Kungfu Master</Title>
<platform>UNIX</platform>
<ggi>1523</ggi>
<dbname>kfmaster</dbname>
<gversion>1.0.0, 2.3.0d</gversion>
</projectdetail>
</project>";
XDocument doc = XDocument.Parse(xmlData);
XElement projectObject = doc.Descendants("ProjectDetail").Where(rec => (string)rec.Element("Title").Value == "Kungfu Master").SingleOrDefault();
if(projectObject != null)
{
projectObject.Element("GVERSION").SetValue(projectObject.Element("GVERSION").Value + ", 3.0.0a");
}
string test = doc.ToString();
doc.Save("path");