How do i change my water fountain valve for my fish pond pump?

i have a pentair aquatics uno pump/filter combo and it comes with the bell fountain but i can't figure out how to get it on my pump.How do i change my water fountain valve for my fish pond pump?it should screw right on and off..How do i change my water fountain valve for my fish pond pump?unga bunga! me not no. sorry to not help you.