First, you need to collect enough medals from the blue sphere stages to unlock Debug Mode.
Then, enable debug mode in the game options menu, select no-save mode, press the start button and hold Q.
Finally, play these songs in the sound test on the level select menu in this specific order: 02, 00, 01, 07, 00, 08, 01, 05.

When you play the last song, you should hear a ring noise. If not, try again.
If you heard the ring noise, select a level and press Q to go into debug mode.
Spawn in an S monitor and break it to make your character turn into their super form. Then, just press the jump button in mid-air.

Have fun!