I'm confused about what network key are you referring to?
If it's the wireless network key, go to network connections, right click the wireless connection go to 2nd tab and you should see the network under preferred list. highlight it and select remove/delete. after it's gone, hit Ok to close the dialogue boxes. Now you need to manually reconnect to the wireless network in question, windows will prompt you for the network key, enter it and if the connection is established correctly you should be good to go.