Q: How do I clear setup information in Apple TV and start over?

I assume you are using the ATV and not stuck in initial setup?

Do you want to change networks or erase everything on it and start from new?

Also what is the model number of your ATV?

I'd pull the power plug and restart it - hopefully that will make it start the process from scratch.

A gentler option might be to reboot using the remote - hold menu and home button for about 6 seconds.

http://www.cultofmac.com/404949/how-to-restart-or-just-sleep-your-apple-tv/

In the unlikely event it doesn't reset you'd probably need to restore via iTunes using a suitable USB cable.

I don;t suppose you can access the menus at all - In Settings there is a rest option, see pics in article above.

At any point in the setup, pressing menu over and over backs you all the way out to language selection to start over.

Now that you stated the problem consider working through this:

Get help with Wi-Fi networks on your Apple TV - Apple Support

Does the network you are trying to connect to have restricted access or is a captive network as described in the above article?

If you router is set for WEP encryption try WPA2 personal instead.