I am afraid that all you have now is a picture (jpg)
You need a fancy photo editor software or a vector graphic one.
The principle which I would follow in outline is to encircle the text you want to change and fill with white (Luckily white is the background). Then type in the text you want and save the result)
There are all sorts of snags along the way, starting with a small picture in Word. Do you know anyone who can do this for you. If you have to go the picture editing way it might be better to scan the old letterhead at higher definition and start with smooth lines and edges.
If you have Corel Draw or Illustrator, you could quickly build this from scratch. There may be cheaper versions of both. Other members may help from their own experience.