Slide your finger down the display starting from the top edge of your mobile phone.
Tap the settings icon.

Tap Networks.

Tap Tethering & networks.

Tap Mobile networks.

Tap Network operators.

Choose one of the following options:
Select network manually, go to 2a.
Select network automatically, go to 2b.

Tap Search networks.
Your mobile phone will search for networks within range.

Tap the required network.
Your display will tell you if you've access to the network you've chosen.

Tap Select automatically.

Tap the home icon to return to the home screen.