Here's a code snippet to edit nodes in an existing InfoPath 2010 Xml document from a selected Form Library in SharePoint 2010:
SPWeb mySite = SPContext.Current.Web;
SPList oList = mySite.Lists["FormLibraryName"];
SPQuery oQuery = new SPQuery();
oQuery.Query = "<Where><Eq><FieldRef Name='FileLeafRef'/><Value Type='Text'>TargetFile.xml</Value></Eq></Where>";
SPListItemCollection listItems = oList.GetItems(oQuery);
if (listItems.Count > 0)
{
SPListItem item = listItems[0];
MemoryStream oMemoryStream = new MemoryStream(item.File.OpenBinary());
XmlTextReader oReader = new XmlTextReader(oMemoryStream);
XmlDocument oDoc = new XmlDocument();
oDoc.Load(oReader);
oReader.Close();
oMemoryStream.Close();
XmlNamespaceManager nameSpaceManager = new XmlNamespaceManager(oDoc.NameTable);
nameSpaceManager.AddNamespace("my", oDoc.DocumentElement.NamespaceURI);
oDoc.DocumentElement.SelectSingleNode("my:NodeToUpdate", nameSpaceManager).InnerText = "Update text";
oDoc.DocumentElement.SelectSingleNode("my:Node/my:SubNode", nameSpaceManager).InnerText = "Update text";
mySite.AllowUnsafeUpdates = true;
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
SPFile oSPFile = mySite.Folders[mwp.FormLibraryName].Files.Add(item.File.Name, (encoding.GetBytes(oDoc.OuterXml)), true);
}