You can click on any item in the checklist then drag-and-drop it to the place of another item.
When all items are checked
When all the items in the checklist are marked, the task does not become completed automatically. You need to make sure you have done all the key steps of the task and then complete it manually.
Delete a checklist
To delete a checklist item, highlight it and press Backspace or Delete
To delete the entire checklist, highlight all the checklist items by holding the Ctrl key and pressing Backspace or Delete