To change a User's password, simply navigate into the User's Profile, click the Actions button, and then choose 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!
When the password is changed, an event will be recorded in the user's Change history, accessible in the main profile window by clicking the Tabs button in the top left corner, and choosing "Change History Tab". The password will not display in this area, but will show that a change was made and by whom.