To enable, change, or disable your 2-Factor authentication, log in and click on the button under "Security Settings" on the Settings page.
Once in the "Security Settings" page, click on the button "Update" and set however you want to receive your 2FA.
You will be prompted to confirm your identity with your current 2FA method.
Once you have entered the code that was sent via e-mail, SMS, or your Authenticator app, you can make changes to your 2FA.
If the above steps do not work, plese reach out to: info.eu@bitflyer.com