There are two way to edit Odoo code:

- The first and the simplest one is editing it trough the GUI. Here you can change the view and add actions on the modules/views.

- The second, is to edit directly python and xml files trough a good IDE like Pycharm. More complicated if you aren't skilled enough but you'll gain full control.

Side note: editing directly the source code of a module isn't a good idea. It's better create a new custom module that inherit a view or a function or for creating a completely new piece of code to suit your needs . Have a look here or directly here. 

If is not what you need, please elaborate a bit more your question and try to add an example.