You mean falling due to gravity?
If you just want it for 1 sprite then doing it manually would be ok, if you want more than just falling then using something like box2d to do the calculations would be a good idea.
To do it manually, you would need to store a sprite speed value and change the speed by gravity(10 m/s sq to make it easy) computed for the delta time.
Bare in mind that this would give you meters per second, you might have to apply a factor to make it look ok on the device.
If you have a tick or update function that looks like
void tick(float dt) //dt is seconds since last call
Then you would do
Then you would update the sprite position by spriteSpeed
sprite->setPositionY.y spriteSpeed.y * dt)
Or something like that… ignore everything I said if you didn’t mean gravity.