Provided you have the necessary permissions, you may reset another user's password by doing the following:
Navigate to System > User admin, in the left pane.
Click on the user whose password you wish to reset.
In the right pane, click the Reset option.
The user will then be sent an email with a link to reset their password.
Hint: A user may reset their own password from the login page by clicking the "Forgot your password?" link.
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