If you want to delete a single training from a series of trainings without deleting the other events in the series, you must proceed as follows:
Go to the event you want to delete.
Click on the pencil at the top to edit the event.
Now go to the bottom and click on "Delete".
Now select "Only this training" and confirm the action.
The training is deleted.
If you want to shorten a series, you can find out how to do that here.