You are saying about Event's Custom Fields? If so, the field event_properties.php is exactly the file you need to modify. The code for rendering these custom fields are from line 321 to 346, not sure how you want to customize it?
OK, great. Once the modification is done, you should move it to PATH_TO_TEMPLATE/html/com_eventbooking/common folder so that the change won't be lost when you update to future releases of the extension