There are three ways to open the Task Editor
1) Select the ROW to Edit and then click the Edit toolbar button
2) Simply double click the row you wish to edit.

3) Select the ROW to Edit and then Right Click the row to display the popup context menu, and click the Edit button
All of the above methods will open up the Task editor, which can be completed in the same manor as it was originally added.
When done, Click Save to commit your changes