First, you need to have more than one user on your computer, and you need to sign into other persons account. Plus the people's account where youre signing in, needs to be admin, only at there you can do this.
Second, use only into good purposes, like if youre forgot your password, you can easily change it this way. But changing other persons password without permission is bad thing, and shouldn't be done by anyone.
Third, this might be old to someone, but remember, that it can be new to others. So dont spam like its old n stuff.
Heres the tut: