Welcome to the forums Larin!
If I understand you correctly, then all you need to do this. In your trigger
activation: Radio Alpha
on activation: whatever you already had; "1" setRadioMsg "New message"
So when somebody uses the radio, the old message will vanish and the new one will appear. If you do not want it to happen immediately use "1" setRadioMsg "null", and then put the new message in later with a trigger.
Hope that helps.