In this world, kill the ender dragon, then take the teleporter, and press skip to skip the credits. This will take you back to the main world. Then, head over to the area in the back where it says update 1.0. In there, open the chest next to the portal on your right. Retrieve some glass bottles. Then enter the portal.
Then, use some blocks to make your way up. Then, go to the center of the large poles. Here's you will find the teleporter. Instead of using it, place an end crystal on the center of all 4 rows of 3 bed rock rows. (teleporter consists of 4 rows of bed rock, each having 1 piece of bed rock in between. Place the end crystals on the center of all 4 rows of 3 bed rocks, so 4 crystals total)
Now the dragon will respawn. Once it is completely respawned, wait for him to shoot some balls towards you. Once these hit the ground, purple/red/brown smoke will evaporate from the ground. Use a glass bottle on this smoke to collect dragon's breath.