3G or UMTS can be a huge battery drain. You don’t need a 3G connection unless you stream multimedia to your phone. A regular GPRS connection is enough for checking emails or browsing mobile friendly websites.
So in order to disable it on your Windows Mobile device, do the following:

1. Tap on the Signal strength icon
2. Select Settings for the Phone Settings
3. In either Services or under the Band tab look for the Select Your Network Type option
4. Select GSM as the default rather than Auto

Via MR Mobile