Method 1:
To Perform this action: Dashboard -> IP Addresses -> Select IP Group -> IP address -> Edit -> Change server signature-> Update.
Method 2:
You can also change the server signature from "domain actions" in your Domain Group, when you click on "Modify" button in the IP Address section.
To perform this action: Dashboard -> Domains -> Select Domain Group -> Select Domain -> Click "Actions" -> click "Modify" under IP Address section -> Change Server Signature -> Update.