A very basic example:
f = figure;
t = uicontrol(f,'Style','text',...
'String','Select a data set.',...
'Position',[30 50 130 30]);
t.String = 'hello World';
This changes the static text from 'Select a data set.' to 'hello World'
Basically, all you are doing is changing the string property in an object that is of style text .
Does that help?