How do I use DataContractSerializer to create nodes/elements for a containing XML file?

Question

Say I am persisting some application data to a single XML file and I want to use the DataContractSerializer to create all or most of the elements. So my file might look like:

<?xml version="1.0" encoding="utf-8"?>
<Settings>
  <A ... />
  <B ... />
</Settings> 

(A and B are serialized using DCS.)

How would I go about writing this file initially, then how would I overwrite it when the data has changed (e.g. replace the A element with an updated instance)? What about reading it back and deserializing each element?