Typical XML parsing and modification like this example:

public void HTTPActivity8_OnBeforeExecuteStepEvent(object sender, STActivityBaseEventArgs args)
{

string strMyXml= this.HTTPActivity8.XMLBody.InnerXml;

// load the XML from the activity
XmlDocument d = new XmlDocument();
d.LoadXml(strMyXml);

//identify and modify the specific element identified as ''model'' in my example
XmlNode root = d.DocumentElement;
XmlNode myNode=root.SelectSingleNode(''model'');

// plug in your value
myNode.InnerXml=''MY MDIFIED VALUE GOES HERE'';

// then save the modifications back to the activities XML
this.HTTPActivity8.XMLBody.InnerXml=d.InnerXml;

}