When an account's password is changed, the account will be logged out of all active sessions.
Resetting your own password
There is a link at the bottom of the login screen ('Forgotten your password'), that will allow you to reset your password.
Enter your email address and a link will be emailed to it that allows you to change the password.
Changing the password of a different account
If you are logged in as an Administrator, you can change any user's password by opening their Account page (in the Accounts section on the left hand side), and clicking Change Password. Type in and confirm the new password before clicking Save.