Step-by-Step
You can use Duo to add a new phone to your account, remove an old phone, or update preferences. Start by going to https://2faenroll.dartmouth.edu (even if you've already enrolled in Duo), or by signing into a service protected by 2-factor authentication using Duo, like bwa.dartmouth.edu.
The authentication prompt will display options for My Setting and Devices, or Add a new device.
Before authenticating, choose the appropriate link to manage your devices. Then you will be asked to confirm that it is really you, using a currently enrolled device. You can't get in to the device management portal if you do not have access to any enrolled devices. In that case, you'll need to contact your department's IT support office for assistance.
Device Management Portal
After authenticating, you'll see the device management portal. This is where you can edit your existing devices or add a new one.
Note: Depending on how many devices you have already enrolled, you may see a scroll bar to the right side of the window. Scrolling will reveal more options you may need to take.
Follow the prompts to manage or add devices, then choose the "Back to Login" button to return to the sign-in for the service. You will need to Duo authenticate to get into that service.