You would need to know (keep track of) the bar number (lnBar) that you want to change. Bars can be added (or text changed) by the following:

DEFINE BAR lnBar OF MenuPadName PROMPT "Some Prompt Text"
ON SELECTION BAR lnBar OF MenuPadName DO SomeCommandProcedure

To remove a bar:

RELEASE BAR lnBar OF MenuPadName