You must have the necessary roles to perform this action.
Go to the Users page on the Main menu > Management >User Management > Users.
Find the user whose password you wish to reset and click their name to go to their user profile page.
On the user profile page click "Edit this profile".
In the "Change password" section click "Reset password". This resets the password and sends it to the user by email. If the user has been locked out due to too many incorrect password attempts their account is also unlocked.