[Help] Edit file without changing any other data in file

I am trying to edit a file; the program should go to a certain address of the file and write a string, overlapping any previous characters in that spot. I’ve tried fs.write, however it removes all the data in the file to write that string. What can I do?

Submitted April 28, 2018 at 06:33PM by 1plus1equalswindow_