How do I change the subject of Reservation Confirmation emails from the Booking Engine?

This setting is found in rezstream.net. You will want to log in and select your property. You will then go to Account>Guest Interface>Step Customizations. 

 From there you will see this box:

This is where you can put in the text that you would like to be the subject. 

For example if you put in "My Property", the guest will receive a reservation confirmation with the subject line "My Property Reservation Confirmation"