How rewrite property in coredata?

Question

How i can

Check that property==nil

ReWrite property

For example

        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
  NSEntityDescription *entity = [NSEntityDescription 
         entityForName:@"Event1" inManagedObjectContext:app.managedObjectContext];
  [fetchRequest setEntity:entity];
 NSError *error;
   NSArray *fetchedObjects = [app.managedObjectContext executeFetchRequest:fetchRequest         error:&error];

     for (NSManagedObject *Event in fetchedObjects) {
      int info=[[Event valueForKey:@"values4"] intValue];
     if ([info!=nil])
      printf("\n%d", 77777);
     NSLog(@"Event1: %@", [Event valueForKey:@"values4"]);
          }       
       [fetchRequest release];

I read property , check if it nil or not , that write or rewrite