Question

what is meant by water exerts upward pressure and how can i prove that

Answer 1

water exerts upward force is known as buyouncy force or upthrust
you can look it in why a object sink in water or another does not sink in water
it is because water exerts same force for same dimension of object but heavy object has more
mass and hence it get sink

Answer 2

that force is called buoyant force
you can prove it by keeping an empty bottle in water. you can see that it will float