You can use your email address or a sign-in ID to log in to your Nintendo account.
Since your email address can be known by other people, using a sign-in ID with a unique character string that only you know can help prevent unauthorized access of your account by a third party.
Please avoid using a character string for your sign-in ID or password that you also use on other services.
A sign-in ID is only used only for logging into your Nintendo account login. For example, you do not need to use your sign-in ID for friend registration. Therefore, please avoid giving it to others.
Complete these steps
Go to the Nintendo Account website and sign in to your Nintendo Account.
Select Sign-in and security settings.
Select Sign-in method.
Click Submit to have a verification code sent to the email address on file.
If the email address is incorrect, click the Email address menu setting under User Info to change it.
Enter the verification code from the email, then Submit.
Set to Sign-in ID only (recommended)
so that you cannot log in with your email address.