Click on Accounts -> Accounts
Select the Account the user whose password you want to reset is associated with.
Select Action -> Manage as Buyer
Select the user and select Action -> Reset Buyer password
A confirmation message appears.
Click "Reset" and the new password will be sent to the user and presented to you in a popup window afterwards.
The new password will be emailed to the buyer. To configure your own branded email template: Customize the Reset Password Email Template