You can use Wi-Fi as an alternative to the mobile network when establishing an internet connection. This way your phone doesn't use mobile data.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Network & Internet.
Press the indicator next to "Wi-Fi" to turn on the function.
Press the required Wi-Fi network.
Key in the password for the Wi-Fi network and press Connect.
Press the Home key to return to the home screen.