How do you unlock the exploding arrows in shadow of the colossus i really want them

You can unlock upgrades in the game for weapons, strenght, ect. by beating the game. You will be asked to save after the end creds and movie. Load the saved file and go to the statues of the Colossi. Pray in front of them to enter "Time Trial" mode. Every victory under the time limit will grant you with a new or upgraded weapon.