For this you are going to need SQLite Editor
Step 1: Open Magic Piano and play the song you'd like to cheat. Doesn't have to be a perfect run just finish the song and remember the score.
Step 2: Close Magic Piano and open SQLite Editor. Find Magic Piano, open it, then open "magic.db"
Step 3: Open "score_info"
Step 4: In "score_info" under the "top_score" section find the score you got on the song you want to cheat.
Step 5: Longpress on that score and change it to the same value under "max_score" for that song, do not set it any higher.
Step 6: Press save, exit SQLite editor and go into Magic Piano and look at the song, the score will be what you set it to and 3 stars.
Step 7: Profit.