I think that
@DonManfred did understand your question, but maybe you did not understand his answer. Here is what I think you want to know ...
At some point in time you issued a StartServideAt for some service. At some later time you discover that you want the service to run at a different time, and you want to know how to prevent the service running at the originally planned time. In other words, you want to cancel the currently scheduled event and replace it with another.
You cancel the currently scheduled event with a CancelScheduledService(serviceName) statement, and then issue your new StartService instruction.
Here is a link that seems to be answering the same question.
By the way, CancelScheduledService will cancel all the pending StartService events, so if you are planning multiple events then you will have to save them in a list and issue fresh StartEventAt commands.