Updating conn string at run time

[venerable bede]

I dont get this.
I have figured out how to read then update settings in my app.config at run time using the following exert:

Configuration config = ConfigurationManager.OpenMappedExeConfiguration(fileMap, ConfigurationUserLevel.None);
ConfigurationSectionGroup grpApp = config.GetSectionGroup("applicationSettings");
ClientSettingsSection secPropSet = (ClientSettingsSection)grpApp.Sections[sSettingsSection];
secPropSet.SectionInformation.ForceSave = true;
 SettingElement elm = secPropSet.Settings.Get(SettingName);
elm.Value.ValueXml.InnerText = NewValue.Trim();
config.Save()

This works fine with custom sections but not with <connectionStrings>.
Reading the connection strings can be done using the following exert :

            //conn strings
ConnectionStringsSection connStringSection = config.ConnectionStrings;
 ConnectionStringSettingsCollection connectionStringsCollection = connStringSection.ConnectionStrings;
System.Collections.IEnumerator connStringsEnum = connectionStringsCollection.GetEnumerator();
int i = 0;
 while (connStringsEnum.MoveNext())
{ ///blah blah

}

For the life of me I cannot figure out how to write an update made against a connection string.