Editing your session
Navigate to your session page
Click on "Update Session" (upper-right corner)
From there, you should be able to update max learners, duration (in minutes), date, time, description, and whether or not you want to allow co-hosting.
Editing your series
Navigate to your series page
On the left-hand side (if you're on your computer) or towards the top (if you're on a mobile device), click on "Edit Series"
Click on "Edit Series" again
From there, you should be able to add and delete sessions, reschedule sessions, edit descriptions, and change series settings.