Assuming you are referring to a Workflow Task, open the Workflow Designer, double click on the workflow to open the graphical editor.
In the button bar at the top, look for the icon labelled Task List. In there you can add and remove fields from the Task List. Note that you must re-publish the workfow for the setting to take affect, no active instances will reflect the change, only new ones started with the new version.
Phil Robson
Senior Director Support Americas