XML : Change xml attribute c#

need to change attribute with name = "SqlServer"

config file just like xml, not from my project

      <add name="SqlServer_WinAuthentication"           connectionString="Data Source=.; Initial Catalog=MeterShop; Integrated Security=True; Application Name=MeterShop; MultipleActiveResultSets = true; Pooling=True;" />        <add name="SqlServer_SqlAuthentication"           connectionString="Data Source=.; Initial Catalog=MeterShop; User ID=sa; Password=xyz; Application Name=MeterShop; MultipleActiveResultSets = true; Pooling=True;" />  </connectionStrings>