The same way you do with the original TCP message system:
•GetNetworkMessageFloat
•GetNetworkMessageInteger
•GetNetworkMessageString
Think of the UDP listener as just another network instance and handle them independently, but manage retrieving their message data the same way. One restriction to currently be aware of is that the CopyNetworkMessage command doesn't yet fully work with UDP messages, only TCP. If you need to copy an entire received UDP message for rebroadcasting or storage (such as for a server-client setup), you'll either need to rebuild it entirely for each client to send to or get every line of data from the message first, then perform the copy operation after. As far as I know though, this has been fixed for the next update so this is only a temporary issue.