Adobe Flash Player

Hi, I am not able to open any videos because I couldn't install the new version of the Adobe Flash Player. The problem is I haven't tried to update it before, I think someone else did the old version so I don't know my username or password to update it. No one knows it who use my computer. How can I learn my password or how can I change it. We don't have any clue about our password or username and is it possible to use someone else's password and username? Please answer. Thanks.