1. Launch the Settings app on your iPhone or iPad.
2. Tap on Wi-Fi.
3. Tap on the info button next to the network name.
4. Tap on Forget This Network.

If the network is not appearing, or the network is not letting you connect, check that there are no pending updates; the IOS device may also need a quick reboot to reset the network process.

If the above is still not resolving the issue, then you may need to completely wipe the network settings from your iPhone/iPad; to do this:

1. Launch the Settings app on your iPhone or iPad.
2. Tap on General
3. Go to the bottom and tap Reset
4. Tap Reset Network Settings

Please note that doing this will wipe all Wi-Fi and VPN connections, and will need set up again.