You cannot modify the contents and settings of your DVD. The DVD you have bought is probably copyrighted which means that you will have to decrypt the DVD, copy it back to the computer and then burn it again.
You can perform decrypting with DVDFab. The software gives the possibility to perform backups as well or you can use IMGBurn to save the DVD as an ISO image which you can modify in UltraISO.
This is the only way to alter settings for the content on the DVD.