I personally feed my arowana cut up pieces of raw uncooked market shrimp; I buy it with shell on, thaw it, remove the shell,cut it up, then feed the aro. You can try pressing pellets into the pieces of shrimp; most keepers would recommend either Hikari or NLS pellets.
If you are lucky, your fish will take to pellets, and then you can skip the shrimp. Otherwise you will have to try starving it to eat pellets and that can be stressful.
If you are unlucky, you could get a picky eater. Sometimes crickets/meal worms/king worms can spoil a fish and it will only eat these live foods.
I am guessing your fish is already on cut up prawn. Ask the previous owner what he was feeding it.
Good luck and congratulations on your new fish.