How to change my password C3700

I need to know how to change my password? Not to access the site but to the system itself....reason is I know that someone is using my service other than me. So, I would like to lock them out. I have no clue if the seleced the correct location. I am clueless as to what that means. Again, my issue is that someone is using my service and I want to be able to lock them out.