Use keys 1, 2, 3, 4, and 5 to change instruments. Use keys A, B, C, D, E, F, G to play the according notes. (If you don't know anything about music, that's fine too.
WARNING: If you have seizures, please do not hold down the E or F keys, or you may experience a seizure. (I don't know much about seizures, but just so you know...)
