Your only option is to restart the game from scratch and try to do better the next time around. There's no way to change their appearances once you've started a game.
Not happy with the way your Scott or Sara Ryder turned out once you started playing Mass Effect: Andromeda? We've got bad news. To do so, use the facial reconstruction station on board the Tempest in the med bay.
Update: Mass Effect: Andromeda patch 1.08 now allows you to modify Ryder's appearance mid-game.
