In Final Fantasy XV in order to upgrade the Auto Crossbow, you need the Cactuar Needle. Take on the quest A Better Crossbow given by Cid in Hammerhead to find this item. This Cactuar needle location guide will show how to get the Cactuar Needle in the game.
The game released for PS4 and Xbox One in Nov 2016. It is an open world environment and a battle system similar to the Kingdom Hearts series.
By talking to the Tipster in Meldacio Hunter HQ, you can get different locations for the Cactuar Needle.
You can find the complete Final Fantasy XV Guide here.