To reset an Exchange+ Mailbox Password, you will need to access the Exchange Panel. Click on the Manage Exchange button from the Console User list for the user's domain you managing:
This will automatically log you into the Exchange Panel, and display the Mailbox Management screen:
Click on the Display Name for the user who's password you wish to reset; This will bring you to their mailbox management screen, where you can navigate to the "General" tab and alter their password. Type in a new password, then click the "Set Password" button:
The new password will take effect in just a few minutes. During the busiest hours of the work week, this change can take up to 20 minutes. You can test it by attempting to log into OWA using the email address and password.