For whatever reason, Microsoft decided to remove the “Manage wireless networks” from the Network and Sharing Center.

As long as you want to manage the connection to a network you are already connected to, that’s no problem. Just right-click the properties of this connection.

Managing a not connected wireless network is possible, but not that simple. You have to become familiar with the netsh wlan command.
To list all the existing profiles, open a command prompt and enter

netsh wlan show profiles

From my point of view, the easiest way to manage an existing profile is to delete it via netsh and then re-create it using the Windows 8 UI tools. To delete a profile, use

netsh wlan delete profile profilename.