If there's a problem connecting to the network, or if you get a password error
On the Apple TV menu, go to Settings > Network > Wi-Fi. Select your current Wi-Fi network and click Forget Network. Press Menu or the Back button on your Apple TV Remote. Then select Network again and select your Wi-Fi network to connect.
If you want to switch from an Ethernet connection to Wi-Fi
Unplug the Ethernet cable from your Apple TV. You might have to wait several minutes for the Apple TV to notify you that you can join a Wi-Fi network.
For more information and troubleshooting see: If your Apple TV can't connect to Wi-Fi - Apple Support