Please note that currently you can add/remove domains within Monitor Backlinks but you can't just change them.

How do I delete a domain in Monitor Backlinks?

First, click on your domain and go to Manage Domains:



Select the domain that you'd like to remove, and then click "Delete":



After you delete the domain, you'll be able to add a new domain if you are under your plan limit (please note that you can do it once per month). 

How can I add a new domain?

You can add a new domain by:

clicking on your domain on the top left
clicking on "Add New Domain" from the pulldown menu that appears: