How to Update or Edit EKEvent in iOS and store in native calendar using identifier?

i am using EKEvent in my app to fetch all store events and i want to edit any particular event by identifier and re-save in existing..so what should i do for that?

EKEventStore *store = [EKEventStore new];
    EKEvent *event = [EKEvent eventWithEventStore:store];

    event.title = @"abc";
    event.notes= @"this is updated notes";

    event.calendar = [store defaultCalendarForNewEvents];
    NSError *err = nil;

    [store saveEvent:event span:EKSpanThisEvent commit:YES error:&err];

this code is for i am using to store event, but where to pass identifier for update only particular event?