If you are looking for how to play freely in Pianist HD: Piano+, here are 2 simple steps to do it. Check it out!
Look at the top menu bar, tap and hold the “Playback” icon on your right side.
You did it! Now you can play freely whatever song you like. You can turn on the "Freeplay Mode" either on the main screen OR while playing a particular song which you don't want to continue anymore.
Noted: How about change into another song? Simply tap "Songbooks" my dear.