Answer: For me, it would probably be the title slides if you can only change one slides.
Explanation: Because the title slides represent your presentation almost more than anything. Change your title slide into something that reflect on your presentation. For example, if you were doing a presentation on wolves, you should change your title slide's background or at least insert a picture of a wolf or something that has to do with a wolf. This way the people that you are presenting to would get a good guess on what your presentation is about based on your background or picture along with the title of your presentation. But if you were doing a presentation on the Greek Gods and your title slide's background is a pencil, people would be confuse. That why you should change your title slide so that it reflects on all slides in your presentation.
