Simply edit the event/training from which the series is to be active, but you don't want it to be active for as long as you originally thought?
No problem, you can easily shorten the series date.
Simply edit the date from which the series should no longer run.
At the bottom you just have to click on "Delete".
Then select "All trainings".
The event you have edited and all future events of the series will be deleted.
Example:
You have the following dates in the series:
Training 1
Training 2
Training 3
Training 4
Training 5
If you want to keep only the first three trainings and the series should end there, you have to edit "Training 4" and delete "All Trainings" there.
This will delete "Training 4" and "Training 5". The other three events will remain.