I'm sorry, but you cant. A bitmap file doesn't contain editable text, it only contains a grid of colored pixels. What you need to do is create a new image. For that, you'll need to know the color and the font.
If you don't have the hex code for the color, you can sample with the eyedropper tool. Since the font is so thin, sample at the thickest area you can find to minimize anti-aliasing color blending.
If you don't know the font, try one of these sites:
(It looks a bit like Century Gothic, but it isn't.)