If you saved the Photostory project then re-open it in Photostory, save the project with a different name, then make the changes saving as you go along. Burn to disc again when you have completed the changes.
If you did not save the project then, while you can get and import the movie file(s) from the DVD you will not have the individual images, video etc on the timeline to work with, just one video file which you will have to cut and edit as you need.