1. Search here or in here for examples of how to replace text in a file.

2. Find out what method has been used to encrypt the passwords in the first place.

3. Put those together in a script, and if it doesn't work, share it with us and we can help.

I'm not interested in writing programs for you (unless you are paying). I delight in helping people learn to program in Perl. The same is true for most of the people here.