a - extract the video and audio files to a digital file (a container with both video and audio and maybe subtitles) that can be easily played-back on your computer using a video player software(specify the software and hardware for playback).
b - make another dvd with the same content as the one you currently have.
Step 1- put the disk in your Mac
Step 2- right click on the DVD's icon and choose "Burn <filename> to disk"
Step 3- do what the computer tells you to next