Hi,

Rather than creating or editing from the user interface it is better to create a new custom module which adds this functionality. Once you applied lot of changes inside a database from the user interface, and later if you need the same in another database or if you need to move it to the other database, it will be not too easy. 

Actually customizing from ui or from the custom module depends on the persons knowledge level, if you are skilled in coding, do it as a module.

Hope this videos will be helpful for you.

1. How To Inherit And Add New Fields To Existing Views
   

2. How To Inherit And Modify Existing PDF Reports in Odoo
   

Thanks