When you have finished testing yourself on one of the 16 skills available on your Pix profile, you can retry the questions you answered incorrectly to improve your score.
To do so, simply click on the "Try again" button that appears on your profile after 4 days. Thanks to this feature, you can try again the questions you didn't answer correctly, while keeping the pix you already earned. This way, you can try to improve your score and move up to the next level.
If you want more practice, you can also reset a skill and start over from scratch.