I assume that you are visiting your old town and the old lady thinks you are her late husband or son, or somebody close to her.
I cannot write a letter for you, but I can tell you what you can write about. You can write about how you are feeling scared coming home after such a long time, but also nostalgic because you missed home a lot. You can also talk about this old lady, or pretend to be her Jim because you may fulfill her last wish to see her loved ones. You can say that doing that made you feel good about yourself, and that seeing her happy filled your heart with joy.
Hopefully you'll find something useful here!