To edit a single subtitle, click on that subtitle in the list.
To adjust the timing, use the “start time” or “end time” buttons, or drag the subtitle in the timeline.
On the timeline, drag the left or right borders of the subtitle to change its start or end time.
Whilst holding the SHIFT key while dragging; the end or start time of the neighbouring subtitle will become connected.
To edit the text, make changes to it within the text field.