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.
A prompt will display requesting confirmation of the password change request.
Clicking OK will send an email to the user 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.
You might also be interested in searching the following: