If you want to start fresh, you can begin a new Conversion Job by clicking the New button on the tool bar, selecting the File\New Conversion Job menu item, or by pressing CTRL+N.
NOTE: Creating a new conversion job completely erases all items currently in the conversion task list.
See also:
Saving and Restoring a Conversion Job
Creating a New 'Conversion Job'
Saving the Log as a Conversion Job