How do I modify a XML Node?

I want to modify the Node my XML File look like this.

<Tasks>
   <Task>
      <Title>Title of the Task</Title>
      <Description>Description of the Task</Description>
      <Done>false</Done>
   </Task>

   <Task>
      <Title>Title of anotherTask</Title>
      <Description>Description of anotherTask</Description>
      <Done>true</Done>
   </Task>
 </Tasks>

I could adress the Node like this:

xmlDoc.SelectSingleNode("/Tasks/Task/Description").InnerText = "My Description";

However I have mulitple Tasks. How do I indicate which is which? I want to change the State of the Task "Done" from false to true.