Many players who have played New Super Mario Bros. are eager to get into the seventh world. If you have a Wii version of the game, this method may work too. If not, then we apologize.
Steps

Step 1. Return to the first world

Step 2. Complete level 1-4 twice

Step 3. Here's what you need to do
at level 1–4 you can get a mini mushroom. If you have already passed this level, then you probably know about it. Get a mini mushroom and don't lose it. Complete the level.

Step 4. The next step is optional, but it will make it much easier for you to get into the seventh world
repeat the steps from the third point.

Step 5. Return to the castle in the fifth world and defeat the plant
Shrink and complete the level, after which you will find yourself in the seventh world.
Advice
- The trick is that the level must be passed by a reduced copy of the character. If you are using fiery powers, simply defeat the plant, shrink and take the key.
- The seventh world can also be reached by unlocking the Cannon in the fourth world and using it. After that, you will fly off to the seventh world.