Access XML file in code behind

I wanna ask about WPF C# and XML connections.

Let's skip that part, when I have to create an XML file. I already have one named "xmlconf.xml". And now I want to access it's values from C# code (without using XAML binding). They also have to be re-written during the runtime.

This is a part of my XML file

<?xml version='1.0' encoding='utf-8'?>
<SettingsFile xmlns="http://ift.tt/1IUn8W0"    CurrentProfile="(Default)" GeneratedClassNamespace="SampleApp" GeneratedClassName="xmlconf">
    <Profiles />
    <Settings>
        <Setting Name="string_a" Type="System.String" Scope="User">
            <Value Profile="(Default)">some_string_a</Value>
        </Setting>
    </Settings>
</SettingsFile>

If you think that you're able to help me but have some misunderstanding, please post comments, so I'll make changes to the post.