You must be at least Super Mario to do this. Near the end of World 5-1, when you see two rows of floating bricks past two Red Paratroopas, hit the rightmost brick of the top row to release a vine. Smash the brick next to it, and then climb the vine. After going through the small bonus area, you will be dropped off on a pipe right next to a Warp Zone that takes you to World Six.