Just blast over it with the new string. I mean, you put the string in there so somehow you know what you put it. So you could just make up a new string
caption1 = sprintf('This is %d', 10);
caption1 = sprintf('This is %d', 99);
If, for some reason you don't know what's there, then you can retrieve it
existingCaption = get(gca, 'title');
Then use strfind() or indexing to parse and change the string elements you want changed.