A ball of mass m is projected from a horizontal surface with a speed u at an angle $\theta$ with horizontal. Then ( $\hat{i}$ and $\hat{j}$ are unit vector along horizontal and vertical)

The change in velocity during its flight is 2$u\sin \theta (-\hat{j} )$

The change in velocity between point of projection and maximum height is  $u\sin \theta (-\hat{j} )$

Average velocity during its flight is  $u\cos \theta ( \hat{i} )$

All the above statements are correct.