Select the services tab.
Then, select the service that you would like to modify the rDNS records for.
From there, navigate to the sidebar on the left of the page labeled actions, and click on "Manage RDNS".
From the Manage RDNS page, you can add, delete and change rDNS records to fit your needs. This page will list all subnets attached to the selected service, just simply scroll until you see the subnet with the address you would like to manage. To add another rDNS entry, click on the green plus symbol to the right of the hostname column. To delete an entry, click the red minus symbol to the right of the hostname column.
