Add time entry from the parent item. If you open the item view for an item with subitems, there is an extra selector, where you can choose the subitem to directly log time on.
When you want to add a time entry for any of the subitem from the parent item, select the subitem from the list.
Note: the time entry is not added to the list of time entries made in the parent item. You will find the work log in the subitem's item view.
Edit / delete time entry
On the bottom of the item view you can view your logged entries. To edit or delete a time entry, press the three dots menu behind a time entry (pictured below). When you choose to edit a time entry a pop-up will appear where you can change the fields of your time log.