In the Settings app, tap Wi-Fi and make sure that Wi-Fi is enabled. Tap the blue information circle next to the network you wish to forget, and then tap Forget This Network.

Detailed instructions may be found below.

1) Open the Settings app.

2) Tap Wi-Fi.

3) Make sure that the switch next to Wi-Fi is in the "on" position (to the right).

4) Next to the network that you wish to remove, tap the information icon (circle with an "i" in the center).

TIP: If you do not see the desired network, tap Edit. Then, tap the information icon.

5) Tap Forget This Network.

6) Tap Forget to confirm.

Outcome

The device will no longer attempt to connect to the wireless network that you removed.