If you meant how to change your weapon's appearance without changing its stats, you do so in the Style menu. Unless I'm mistaken, every new piece of armor and weapon you obtain adds it's appearance to your pool of interchangeable.. appearances. So, you can obtain a newer, better weapon, equip it, but keep the appearance of your older, better looking weapon.
The funny thing is, I usually love hunting for gear for new appearances in other games, but in this case I was glad that I was able to keep my original one without sacrificing stats. Now if I can just get a 2-handed sword...