This is only for very advanced developers who want to change some core features! Not for regular users!
If you need paid customization services please send request here:
You should understand codeigniter MVC: https://codeigniter.com/userguide2/overview/mvc.html
Also check some articles in knowledge base like: http://iwinter.com.hr/support/?p=1288
So usually estate_m.php (method save) and controller estate.php method edit
Basic field details are defined in:
affected fields here: