This achievement is related to changing the cosmetics related to an item (either weapon or armor) using the Blacksmith found at Mt. Tianzhushan. (The hub world that gets unlocked after completing the 4th story mission). Once you speak to her you can pick "Decorate" and then basically swap out how a item looks to another item's aesthetics. Note that Decorations costs a tiny amount of Copper.
Surprisingly: there is a video that I found that goes over the ez process (Credit to FP Good Game):