Luckily for us, it's pretty easy to respawn the Ender Dragon. You will need 4 x End Crystals to spawn him. These can be found on top of the obsidian pillars in The End, or crafted by combining the following:
- 7 x Glass
- 1 x Eye of Ender.
- 1 x Ghost Tear.
When you have four End Crystals, go to The Nether and locate the Final Portal. Place the 4 End Crystals on each side of the Final Portal, and upon placing the 4th End Crystal, the Ender Dragon will repsawn. There's a little bit of a wait while he respawns, but you get to see a cool animation while waiting.
And that's all you need to do in order to respawn the Ender Dragon. Good luck with fighting him again!