I´d like to use yyyy-MM-dd date format system wide consistently. In menu “settings>> regional settings” I can change my locale’s default format into any other locale’s. But I do not want a different default, I want to change the locale’s default formatting. Has someone a suggestion how to do so?
You can’t change the default formatting of dates of an locale, because they are just standardized presets.
You can however choose to format your dates using either another preset (locale) or use a custom setting…
To learn how to do those see the link @Aragorn gave above this reply.
In KDE you can modify it by going to: System Settings -> Regional Settings -> Region and Language