There are a couple of apps in the Play Store that do this directly. The user interface to do this by hand can be found here, among other places (that article references the RAZR, but the procedure is identical).
As mentioned previously, LTEonoff App in the Google Play Store will allow the phone to go from 4G to 3G. On the Bionic, change the setting to "CDMA auto (PRL)" and you will have forced 3G only.