In order to change your sprite during the game, you need to level up with points (ex. leveling up your pokemon, beating a gym,battling trainers,walking for loong segments of time...)
to change your sprite offline you either need to: 1. choose wisely at the beginning of your save file. 2. reset your game and choose a different sprite
(P.S. i would reccomend switching to first person in the settings so you dont see or worry about what your sprite is either way)