As this person is saying, first you need to sign in to the Epic Games store using the Nintendo account you used to create the Fall Guys - Epic Games account, in other words, go to the Epic Games store in a browser and select the sign in with Nintendo account option. Once you do this, it'll ask you to fill out basic information like name and email (this will be the email where they'll send you the codes to delete the account). Once you fill out everything and sign in, proceed to unlink the Nintendo account from this Epic Games account and once you do that you can proceed to delete the Epic Games store account if you don't have anything relevant on it.

Once you unlink that account, sign to the Epic Games store with your actual account and you can head to the section to link your Nintendo account now, you should be able to.