Find "Network operators"
Slide two fingers down the display starting from the top edge of your phone.
Select one of the following options:
Select network manually, go to 2a.
Select network automatically, go to 2b.
2a - Select network manually
Press the required network.
Your display will tell you if you have access to the network you've chosen.
2b - Select network automatically
Press Choose automatically.
Return to the home screen
Press the Home key to return to the home screen.