From a Home screen, navigate: Phone icon > Menu (Located in the upper-right) > Settings.
Tap Preferred network.
Tap the appropriate setting:
When LTE/GSM/UMTS is selected, if presented:
Tap one of the following options:
When selecting Manual, tap the preferred network.
The preferred global carrier is automatically selected. You can manually select an alternate carrier but it may require as many as six registration attempts with that carrier prior to successful registration.