How to add new subtitles to a movie which already has them?

I have downloaded a movie and i want to replace its old subtitles with
new ones ( self made ) in my native language . I used notepad but it
subtitles were just on each-other and i couldn't read anything .
I was wondering if there is a way to put some kind of stamp on them
and obscure them and then write on it . Not sure how it can be done .
Maybe there is a program that helps for this thing ?
Thank you :) .