NSMutableArray替换对象

我想要在NSMutableArray替换一个数组对象，从_alertsArray获取了oldArray，然后修改值，用newArray替换oldArray，但是没实现。

NSArray *oldArray = (NSArray *)[_alertsArray objectAtIndex:[indexPath row]];
NSMutableArray *newArray = [[NSMutableArray alloc] initWithArray:oldArray copyItems:YES];
[newArray replaceObjectAtIndex:3 withObject:@"YES"];
[_alertsArray replaceObjectAtIndex:[indexPath row] withObject:newArray];

NSMutableArray *newArray = [_alertsArray mutableCopy];
[newArray replaceObjectAtIndex:3 withObject:@"YES"];
_alertsArray = newArray;