XmlDocument xliffDoc = new XmlDocument();
xliffDoc.Load(filePath);

XmlNamespaceManager xmlNSS = new XmlNamespaceManager(xliffDoc.NameTable);
xmlNSS.AddNamespace("st-xmlns", @"urn:oasis:names:tc:xliff:document:1.2");

foreach (XmlNode transUnit in transUnits)
{
  //Change innerXML while keeping the structure
}
xliffDoc.Save(filePath);

.....
<trans-unit id="17645" identifier="5df0c6519f50ac44d2e9c94c1cf841d4" approved="yes">
  <source>LDL ("Bad" Cholesterol)</source>
  <target>LDL („Schlechtes“ Cholesterin)</target>
  <note>Context: me.body.ldl.text</note>
</trans-unit>
.....

.....
<trans-unit id="17645" identifier="5df0c6519f50ac44d2e9c94c1cf841d4" approved="yes">
  <source>LDL ("Bad" Cholesterol)</source>
  <target>LDL („Schlechtes“ Cholesterin)</target>
  <note>Context: me.body.ldl.text</note>
</trans-unit>
.....