In an Among Us game, creating and setting your character is quite easy. Simply click the ENTER NAME item on the Online menu before the round of Among Us and update your name there. Every new round of Among Us allows for this, but not an already-played game.
Prior to the round, you can also change the appearance of the Among Us characters using your available cosmetics. Just head to the Wardrobe, which may be found in the Lobby or Main Menu.