Yes, you can change your IP's server signature when you are in your IP Group. 

Method 1:

Go to "IP Addresses", then click on your "IP Group", find the IP address for which you wish to edit the signature. Click the edit button next to the server signature details for the IP and a small popup will appear. There you can choose a signature from a dropdown & click update, once you've chosen the new signature.

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.