As far as my understanding, In SharePoint on-premise version of SharePoint corev15.css file is located at c:\program files\common files\microsoft shared\web server extensions\ _layouts\15\1036\styles\corev15.css path in local system.
But I was trying to find corev15.css file in office 365 in SharePoint online, and I could not find this file in any folder. I can check this file in browser by entering correct URL in browser, But I can not make any change as far as I could not open it by physical location.
One of the my office mate advice me to copy the content of corev15.css file into another file and upload it to style Library and overwrite all css again. Is it correct way to do so?