1. Find "Carrier"
2. Choose option
Select network manually, go to 2a.
Select network automatically, go to 2b.
3. 2a - Select network manually
A list of available networks will show on your display.
Press the required network.
Your display will tell you if you've access to the network you've chosen. If you don't have access, it's because your operator doesn't have a roaming agreement with the operator in question.