Joe Dever's Lone Wolf Console Edition

I got this at the first merchant I found. The merchant was found as part of the story in Rockstarn. When you interact with a merchant, along the top left is 4 options. These are Buy, Sell, Upgrade and Inventory. Go into Upgrade and select your weapon. It will tell you at the bottom the required materials and the cost. To fully upgrade my weapon, I had to do it three times.

I had the money and material for the first 2 upgrades, but not the third. Luckily the merchant sold what I needed under the Buy tab. I had to sell a weapon I wasn't using to get the money needed to buy materials and the upgrade (you can continue on with the story and get the option to go back to the merchant once you have more stuff to sell or craft with). Upon purchase of the final upgrade, this trophy unlocked.

You may wish to wait until later in the game when you get a better weapon before doing this.

There is a trophy for upgrading a Sword, Axe and Mace. So you may want to bare that in mind for doing throughout your playthrough