How to change the difficulty

Difficulty controls how hard mobs hit, whether they spawn at all, and whether hunger can kill you.

Change it

1. Open the Settings tab.
2. Set Difficulty.
3. Save, then Restart.

You can also change it live from the Console without a restart:

difficulty peaceful
difficulty easy
difficulty normal
difficulty hard

What each one does

• Peaceful no hostile mobs at all, and they despawn immediately. Health regenerates and hunger never drops below the point of starving.
• Easy hostile mobs spawn but do reduced damage. Hunger can empty but will not kill you.
• Normal the default and what most servers use. Full mob damage, and starvation takes you to half a heart.
• Hard mobs hit hardest, zombies break down doors and can summon reinforcements, and starvation can kill you outright.

Peaceful is not a way to protect builds. It stops mobs spawning, not other players. For that you want PvP off and a claims plugin: see How to turn PvP on or off.

Locked difficulty

If difficulty appears stuck, the world has difficulty-locked set, which single-player worlds sometimes carry in. Run this from the Console:

difficulty normal

If that is refused, the lock is in the world data and has to be cleared before the setting will take.