Once you have enrolled into Duo, you can add, remove or switch default devices using the Duo Self-Service portal. Make sure you have at least two devices registered in case your default device becomes unavailable.

When you log into a system that requires two-step login, your default device in Duo Security will ring, send a text or a push notification for you to approve the login finishing authentication process.  

If you do not have this default device available,  you will not be able to log in.  

To ensure that you have a default device in hand or to switch default devices go to Add or Manage Devices.