A phase parameter can only be edited if Overrule Phase Parameter is set. See Overrule a phase parameter.
Choose Production Management > Resource Types and select the resource type in the Navigation Pane.
Do one of the following:
To edit a phase parameter which is added to a template phase, select the template phase under the tab Template Phase in the Data Pane, and go to the sub tab Phase Parameters.
To edit a phase parameter which is added to an operation, select the operation under the tab Operations in the Data Pane, and go to the sub tab Phase Parameter Overview.