Then you have to delete the old ones off the server, by deleting the events themselves through the app. Im not sure exactly how your calendar app works as i use Outlook, but it should be similar, in that events can be created, made to repeat on schedules etc. Usually to delete an event you just click into it and there is an option to delete it there, or to stop repeating it, or to change it.
If you are struggling to delete it, maybe you can edit the old one and put it in the new details, rather than creating a new one.
