There are several ways to do this. First is to play a song you did not do well on or feel you can do better on, go into practice mode and then do better when you hit perform from the after practice options. Second is to pick a song you haven't played, intentionally do poorly, go into practice mode and then actually try on your second attempt from the after practice options.
Note: You must beat your high score on ALL difficulties. For example, if your high score is 2 million on easy and 1.4 million on hard, you must beat 2 million after practice, even if you are playing on hard.