How can i override Civicrm /CRM/Utils Files?
First of all, you should really customize CiviCRM by developing extensions. There is a developer guide that will explain you how this is done: https://docs.civicrm.org/dev/en/latest/

If you do want to override anyway, you should first of all navigate to Administer>System Settings>Directories to provide the path to the PHP files, have you done this? If you have it should work but I would still recommend to take the extensions route as this is by far the most used and supported route.