If you're trying to update your Community avatar/profile picture, you can choose one of the available ones once you reacha certain rank. To choose your preferred one, just headhere.
Please note that uploading your own profile picture in the Community is a Rock Star perk at the moment.Check out the Rock Star Program page for more details, including how to join.
On another note, if you're trying to change your profile picture in the Spotify app, you can follow the steps here instead. Keep in mind that this can be done in the desktop app, which you can download here.