Your site users can reset their passwords by:
1) Clicking on the "Reset Password" link on the login page.
2) Logging into (B2B site), or go to the storefront (B2C site) and selecting the "My Profile" option (make sure when you are setting up your site navigation that you add this option). Based on the skin you are using this could be under "My Account" link.
Please note: You cannot reset your customers password via. Impersonation. If you would like to reset your customers password for them you must do it through admin by going to the customer account through the site user area.