Re-dubbing a movie on DVD

[shawman]

Hi folks,

I am a bit of a novice, so expect that I know very little.

I need to change the soundtrack on a DVD movie, which is currently has an Italian dub.

I already have a .ac3 file of the corresponding English dubbed sound track.

Any ideas? or
does anyone have some instructions, how I could replace the Italian soundtrack with the English one???

Appreciate you taking a look at this question.

Regards.