You can change what customers can select when answering the question about "How You Heard About Us" option on the booking form.
Navigate to the settings option, and select the 'Branch Setup' wizard.
Cycle onto the 4th step - "Booking Details".
Scroll down to the option called 'How You Heard About Us'.
To add options, type in what you want customers to see, e.g "Google".
To remove existing options, click the 'X' on the option you wish to delete.
Click Save at the bottom of the box.