How to add XML node using C#?

i generate dataset to xml using GetXml() function.

string xml = DS_Test.GetXml();

Now i use

xml=xml.replace("<&ltnode>" , "");

above code to use replace the node empty.

This xml have above 2500 lines. now i want to add one node in first and last how to add.means add the node to start of the xml and the node end of the xml.how to do this.

What I have tried:

i use replace method to replace the xml node how to add the new xml parent node.