Automatic change in a pgn file

[Patatov]

Hello,
I have a very large pgn file with several thousand of games.
With a text editor I'm trying to find a way to change the 6th character after a particular character string.
i.e:
[White "abcdef"]

I want to automatically change it to

[White "abcdefg"]

The line I want to change always start with "[White " and the following character string is always different but always with 6 characters and I want to append the letter "g" to it.
The replace option in my text editor is pretty basic so is there any easy way to do that? or any good Text editor that can help here?

Thanks

[Vinvin]

Textpad can certainly do that for you :
http://www.textpad.com/download/index.html