I modified my subscription but the update is not shown in my calendar application.

When you update your subscription at WebCal.Guru, the new settings take effect immediately.

However, your calendar application does not know about your changes. And there is no way for WebCal.Guru tell your application.

Some applications allow a manual update. Please take a look if you are in a hurry. There's one notable exception - there is no practical way to make Google Calendar to refresh. The best approach is just to wait until the following day.