Hi,

If I get your question correctly you need to update all incidents with the same assignment group. If yes then please try with the below script.

var inc = new GlideRecord('incident');
inc.query();
while(inc.next()){

inc.assignment_group = "d625dccec0a8016700a222a0f7900d06";

inc.setWorkflow(false); //Disables the running of business rules that might normally be triggered by subsequent actions.
inc.update();

}