How to Switch from 4G to 3G Galaxy Epic 4G SPH-D710?

I have received Galaxy S2 Epic 4G SPH-D710 unfortunately, 4G connection is not available where I live Just only 3G is available. When I turn on 4G hopefully It maybe switch to 3G automatically I get that "x" below 4G logo in statusbar which means 4G is not available, I couldn't find any option in settings related to 3G. Do you have any idea on how to set 3G connection on my phone? Is it about installing something so I can get working 3G connection? Thank you :)