Use one to sign in
On the two-factor screen, choose Use a recovery code. Enter an unused saved code and choose Verify and sign in. These codes are not six-digit authenticator codes, so use the recovery text field.
Generate replacements
In Profile → Two-factor authentication, use Generate new recovery codes. The screen asks for a current authenticator or unused recovery code and, where required, your current password. Save the new codes when shown; do not depend on an old saved set.
Turn off two-factor authentication
Use Disable two-factor authentication in the same settings screen and supply the requested verification. The screen explains the session change after disabling.
No authenticator and no codes
Contact support. Account recovery may require identity verification; a support request is not an automatic bypass of two-factor authentication.