Begin by heading to the Task Definition Manager in the side menu 

Find the desired definition under it's respective operation, and proceed to expand it's row (#1) and the row of the task you wish to edit (#2)

Now you can choose to either edit the definition, delete it, add a new section, edit or duplicate an existing section (#3) or modify the elements within a section (#4 - expand the section to modify the elements within that section)

To edit the task definition's name, operation, description (or other attributes), click on the pencil icon beside the task name, and the modal below will appear. While the modal and it's contents will change, the same procedure for editing applies to the task definition's sections and elements. To finalize your changes, click Save.