Q: My network has changed from a WEP to a WAP2. How do I change my IPOD Touch 4th gen

You meant WPA2?

1. Settings > Wi-Fi > select_your_network > Forget This Network

2. Now choose Other and key in your network and the authentication method used then key in the correct password

Try Settings > General > Reset > Reset Network Settings.

Now do not use the network popup list, use Other and put in the network name yourself.

Read here: Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

Does the iOS device connect to other networks? If yes that tends to indicate a problem with your network.

Does the iOS device see the network?

Any error messages?

Do other devices now connect?

Did the iOS device connect before?

Try the following to rule out a software problem:                

- Reset the iOS device. Nothing will be lost

Reset iOS device: Hold down the On/Off button and the Home button at the same time for at

least ten seconds, until the Apple logo appears.

- Power off and then back on your router

.- Reset network settings: Settings>General>Reset>Reset Network Settings

- iOS: Troubleshooting Wi-Fi networks and connections

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

- Wi-Fi: Unable to connect to an 802.11n Wi-Fi network      

- iOS: Recommended settings for Wi-Fi routers and access points

- Restore from backup. See:

iOS: How to back up

- Restore to factory settings/new iOS device.

If still problem and it does not connect to any networks make an appointment at the Genius Bar of an Apple store since it appears you have a hardware problem.

Apple Retail Store - Genius Bar