To do this your image must be digital and still in the imaging computer. Your imaging software should be able to do this if the image was not finalized. If the image was finalized, or permanently saved, then the image will need to have another scale marker overlaid onto the image via image analysis software. You should be able to overlay the new scale marker by overlaying a line, of the correct length onto the old scale bar using either a black or white background to cover the old scale marker.