Pulling out, when you can, and what the host is told.
Open the event and use Withdraw on your own entry. That is the whole thing — no confirmation from the host, and no waiting.
While the event is still going ahead. You cannot withdraw from an event that has been completed or cancelled, because there is nothing to withdraw from.
You can withdraw:
A withdrawn entry stays as a record that you entered and then pulled out. Deleting it would make it indistinguishable from never having entered at all, which is not the same thing — for you or for the host.
You will still see your own Withdrawn status on the event page. Nobody else does.
Withdrawing does not lock you out. If registration is still open you can enter again.
You do not need to do anything. Cancelling is the host's action, it emails everyone who entered, and it cannot be undone.