You can use Miitomo to change your Nintendo Account Mii!

[SonytendoAmiibo]

d21lewis said:
Awesome. No wonder all of my old Mii stuff was there. I used to be addicted to Miitomo.

So, anytime you use the Mii editor from Miitomo on your phone and save the Mii it looks like it automatically uploads to your Nintendo Account. No Switch required. Nice.