第一種:
collectionView.contentOffset=CGPointMake(0, 0);
第二種:
[collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:0] atScrollPosition:UICollectionViewScrollPositionTop animated:YES];
第三種:
[collectionView scrollRectToVisible:CGRectMake(0, 0, 1, 1) animated:YES];