Edit text in a program

There are many days that I'm trying to make changes to a text file
(. Txt) through the program. So I thought of creating a temporary file, but I
do not know what to do next. Suppose that my chain is "Hello the friends" and
indexOf with, I decided to change "the" to put "my". So in the text file,
I should see "Hello my friends," but I tried to get this result, but without
success.

Thank you for your help

What have you tried? We can't help you if we can't see what you've done right and wrong.

edit : subjet solved