Editing a previously made video

A previous employee was asked to update an old company video with our current staff and videos of our facility. After he left we realized most of the changes he made were incorrect and I need to go in and fix them but I am having trouble with Premiere pro. When I try to create a new project pieces and things from other old projects are in the program and sequence sections and I do not know how to clear them without removing the pannel. I also do not know how to take the old video which is one full clip, remove and add in new clips and remove some audio while keeping other audio. I've been watching online tutorials but the way the premiere pro is set up does not seem to match with others in their videos.