How to change space between cells in UICollectionView?

Go to the storyboard , right click on collection view and

And Check your minimum spacing between the cell and reduce it to zero .

//Objective c
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
        return CGSizeMake((collectionView.frame.size.width/3)-20, 100);
    }

// Swift 2.0

func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
    return CGSizeMake((collectionView.frame.size.width / 3) - 20, 100)
}

// Swift 3.0
override func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    return CGSize(width: CGFloat((collectionView.frame.size.width / 3) - 20), height: CGFloat(100))
}