https://www.gamespot.com/articles/unlock-eight-alternate-box-art-options-for-octopat/1100-6460400/

Octopath Traveler's box art celebrates the eight-person traveling troupe in its entirety, but if you find yourself especially fond of one character, Nintendo has provided additional box art you can use instead. The box art is limited to North America.

There are eight designs in total, one for each main character. All eight box arts can be downloaded from My Nintendo for 50 Platinum Points. You can buy them right now. They'll remain on My Nintendo through the rest of 2018 if you'd rather wait until after you've beaten the game though.