To change a User's password, simply navigate into the User's Profile and select Action, Change Password.
When you click that, you'll now have the ability to select your own password, or generate a secure, jumbled password that contains various letters, numbers and symbols.
Once you have a password entered, select whether or not you'd like an email sent to the User informing them that their password has changed. Please note that this email will NOT contain their password, but will merely inform them that the change has been made.
When you're done, click Set Password, and they'll be good to go!