% Create a msgbox
h = msgbox('Hi');
% Change just the message
set(findobj(h,'Tag','MessageBox'),'String','How are you?')
% Make the pushbutton invisible
set(findobj(h,'style','pushbutton'),'Visible','off')
If the user closes the message box then you should create a new one.
On the other hand you can just create a modal msgbox that follows the first one and replaces it:
msgbox('Hi');
msgbox('How are you','modal');
