To change the SOAP protocol version, use the SoapHttpClientProtocol.SoapVersion property.
A ASP.NET 2.0 web service class inherit of SoapHttpClientProtocol class, so you can use the SoapVersion property.
3 values can be used…

MyWebService service = new MyWebService();
[…]
service.SoapVersion = SoapProtocolVersion.Default;
service.SoapVersion = SoapProtocolVersion.Soap11;
service.SoapVersion = SoapProtocolVersion.Soap12;