How to change appearance in Nioh 3
Pcgamer writes that to change your appearance in Nioh 3 you must first defeat the Jakotsu-baba boss in the Hamamatsu Castle Crucible to unlock the Eternal Rift hub area. Once unlocked, fast travel to the Eternal Rift, head upstairs in the building on your left, continue into the room past Saito Fuku, and look for the mirror on top of a box with a cushion in front of it.
Interacting with the mirror gives three options: change appearance, equipment appearance, or transform (the transform option is not available until later). The change appearance option lets you tweak hair, body, and face, while equipment appearance is a gear customisation system that allows you to transmog one item for another visually.
You can also set different looks for your ninja and samurai forms by unticking the 'Same appearance for both styles' box in the Change Appearance menu.