HTC One

Slide your finger down the display starting from the top edge of your phone.

Press the settings icon.

Press Mobile data.

Press Network operators.

Select one of the following options:

Select network manually, go to 2a.

Select network automatically, go to 2b.

Press Search networks.

Press the required network.

The display will tell you if you have access to the network you've chosen.

Press Select automatically.

Press the Home key several times to return to the home screen.