If you mean in Creative Mode, you double-tap the space bar. Or hold down left shift and or press the s button
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Shift
In creative mode, when you are flying, you hold space to fly up and shift to fly down. That is, if you haven't changed the controls. If you did, the key you use to sneak is the same key to fly down.
Inf flying, unlimited resoruces and godmode.
You double tap space to begin flying. After that you can press space to go up, and shift to go down. Double tap it again to get out of fly mode.
You need an a jar from alpha stages, which you can get with Minecraft Nostalgia.