how to delete an existing account?

You can delete yuour Apple ID and everything associated with it. Please read this page to make sure you understand the implications:

https://support.apple.com/HT208504

Particularly note that all your subscriptions in iTunes and yoiur ability to login here will be cancelled, and that once completed the deletion cannot be reversed.

If you are talking about deleting only your iCloud account and not your entire Apple ID, basically you can't: but you can stop any payments for increased storage by downgrading to the free 5GB level:

https://support.apple.com/HT207594

and you can of course remove all the data from it and sign out in System Preferences/Settings.

Login at https://privacy.apple.com/ to permanently delete your account and associated data from all Apple apps and services.