First thing to know is that Deleting a Task is different to Closing it.
Deleting a Task means that it will be deleted forever. There is no undo.
Closing a Task means that you still keep everything to do with the Task but it just moves to a Closed state.
You can't delete a Task if it has activity on it. For instance, if you have Time logged against a Task then you can't delete it. You can however close it.
STEP 1
Locate the Task that you wish delete and open up the Task sidebar
STEP 2
So long as there isn't any activity on the Task you will see a Delete button with a trash icon. This doesn't appear if there is activity on the Task meaning you can't delete the Task.

STEP 3
Click on this Delete button and the Task will be deleted immediately. Be careful as this cannot be undone.
