In World 2-1, partway through the level you need to go down a pipe with a red down-pointing arrow on it (you will see another similar pipe to the right of there, with an up arrow). Whilst standing on it, press Down and the Jump Button together. You will glitch the game and warp to the top-left bit of World 1-1. Flutter Jump to the right as much as possible whilst falling (you'll die if you don't) and you'll fall on part of World 1-1. You can finish the level from there as you would 1-1, with the benefits of being able to gain extra Red Coins, Stars, and Flowers from the level; further, the score earned will be counted towards 2-1 and it will also count as you finishing 2-1!