It depends on the LED screen.
You need to investigate its documentation. Most likely there will be some commands that you can send to the screen. And there has to be a possibility to tell the screen what text it should display. Find out what the screen expects at its serial interface.
Try to connect via Windows' HyperTerminal or some similar program. This way you can play around a little with commands and text lengths and so on.
When you know what you want to send,
the serial port class[
^] is what you need to do it programatically.