To edit a lesson series in Scholaric, is like editing a single lesson, with a few differences.
Click on the lesson title to bring up the lesson dialog. If the lesson is part of a series, you will notice an additional checkbox, to update the series:
Check the Update Series checkbox to indicate that the series should be updated, instead of the individual lesson. (Note: like series deletion, if you apply to the middle of the series, only the lessons from that date forward will be impacted.)
This populates the description field with the original series description:
Note that you can still use the down arrow to see the multi-line description, even if the original lesson had a single line: