Try using the CLABEL function to get the text object handles and then set the text object "String" property to the text you want. Something like this:
[C,h] = contour(peaks(20),3);
th = clabel(C,h);
set(findobj(th,'String','0.59778'),'String','Base Line')
where '0.59778' is the numeric label of the line whose label you want to change and 'Base Line' is the label you want that line to use.
