Can I alter DVD subtitles?

[Morty]

I bought a DVD (its original language is Japanese), and you can turn the subtitles on or off so they're not embedded in the video. The English is *obviously* a fansub, they are WELL bad, so I was wondering if there was any way I can get into the DVD and fix them myself?

(additonally, the DVD itself is badly made, so it's either just crappy Japanese manufacturing or it's bootleg.)

 

[Also in trouble]

I'm having the same problem with a Korean drama. I've seen the show before, and I still can't follow what's going on because the subtitling is so bad. Is there any program that I can download onto my Mac to fix this?