To do so, open the corresponding booking by using the search feature or the booking overview and click "Send order confirmation" on the bottom right. A new window will open where you can confirm sending and also preview the email.
Furthermore, if you want to change the email address of the customer, you can do so in the corresponding booking. Simply click "Edit" and change the email address for this booking. Once you have completed this, you should once again manually send a booking confirmation.