how to remove an Association with External Group

I want to remove an External Group Association from a group.
(Actually we want to associate a different external group)
The clear button is disabled.
What are the steps to replace the external group association?

You could try to reassign the groups with an appropriate toolbox script.

To do this, please go to folder [install_dir]/CentraSite/utilities and run command CentraSiteToolbox.[cmd|sh] ReassociateGroups.jar

Use then the online help to get information about the subcommands and their parameters.