如何动态更改web.config中的连接字符串

ConfigurationManager.ConnectionStrings["ConnectionStringName"].ConnectionString = "Data Source=newServerName;Initial Catalog=newDatabaseName;User ID=newUsername;Password=newPassword";

  <add name="ConnectionStringName" connectionString="Data Source=serverName;Initial Catalog=databaseName;User ID=username;Password=password" providerName="System.Data.SqlClient"/>
</connectionStrings>

ExeConfigurationFileMap fileMap = new ExeConfigurationFileMap();
fileMap.ExeConfigFilename = Server.MapPath("~/connections.config");
Configuration configuration = ConfigurationManager.OpenMappedExeConfiguration(fileMap, ConfigurationUserLevel.None);
configuration.ConnectionStrings.ConnectionStrings["ConnectionStringName"].ConnectionString = "Data Source=newServerName;Initial Catalog=newDatabaseName;User ID=newUsername;Password=newPassword";
configuration.Save();