- What exactly are you trying to do? What exactly is "locked", I mean: what error message do you get when you do it?

A burned DVD is read-only. To change the content, you copy all files to your PC, do whatever you like to do, and reburn the final result to a DVD again. Note that burned DVD's don't have eternal life, so be sure you have 2 backups and refresh these each 5 years.