Open the ICSx⁵ application and tap on the calendar you wish to refresh.
When you update a calendar subscription, this change is not reflected immediately in your calendar. You can just wait from one day or, in worst cases, one week.
There is no direct way in ICSx⁵ to force a refresh. But you'll get the same effect by toggling synchronization for this calendar first OFF and then back ON.
Back at the starting screen, you'll see that synchronization is disabled.
Tap on the calendar top re-open it.