If you mean how do you change passwords for logging in to various sites like this forum, then you don't.
You need to log in, on whichever sites you use and use change the password option on each one.
on my pc say when i log into facebook, it asks me would i like for internet explorer to save the password and i select yes and when i go to log in all i have to do is type the first letter of my user name and it put my user name in and then i hit tab and it automatically puts the password in
Yes that's how it works if you get Internet Explorer to save it so you don't have to remember it, however to change it you need to log in to facebook or whatever site you want to change as I explained in my previous post.