Programatically update connectionstring using C#

You can do the same using the configuration-specific parts of the .NET Framework to save some of the hard work:

// Get the configuration file.
System.Configuration.Configuration config = 
  ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

//Initialize the Connection string builder
SqlConnectionStringBuilder sqlBuilder = new SqlConnectionStringBuilder
{
   InitialCatalog = "YOUR_DATABASE",
   IntegratedSecurity = true,
   Password = "YOUR_PASSWORD"
};

// Add the connection string.
config.ConnectionStrings.ConnectionStrings["CONNECTION_NAME"].ConnectionString = 
       sqlBuilder.ConnectionString;

// Save the configuration file.
config.Save(ConfigurationSaveMode.Modified);

It'll achieve the same result provided you have the right permissions, only you don't have to treat the config file as a generic XmlDocument and parse the items to find the correct section.