Saving information with XML in C#

Question

Okay, here's the deal. I'm writing a program in c# that requires the user to enter information into textboxes. I want the user to be able to click a button and the information is instantly saved into an XML file, to be loaded later. I've found some tutorials on doing this, but the problem is that the ones I found simply add new nodes to an xml file. For example if we have the XML file below:

<login>
  <user>Bob</user>
  <pass>password</pass>
</login>

I want to be able to change the information for <user> and <pass>. The examples that I found all added new nodes onto the file and didn't edit the already existing ones.