The server instance should not be running for it to be deleted.
Click Configuration tab to view the list of available configurations.
Select the configuration from the configurations list.
Click Instances sub tab.
Select the instance from the list of deployed instances under the Nodes section.
Select Delete Instances from the action drop-down list to delete the selected instance.
For deleting a server instance through CLI, execute the following command:
wadm> delete-instance --user=admin --password-file=admin.pwd --host=serverhost --port=8989 --config=config1 serverhost
See CLI Reference, delete-instance(1).