Minecraft: change version, gamemode and difficulty

Change the version: open the Startup page, set the Minecraft version/build and restart — the server downloads the new jar. Back up your world first when jumping several versions.

Gamemode & difficulty: open Files → server.properties and set, for example:

• gamemode=survival (or creative / adventure)
• difficulty=normal (peaceful / easy / normal / hard)
• pvp=true, max-players=20, online-mode=true

Restart for changes to apply. You can also change these live from the console with /gamemode and /difficulty.