You can easily edit your Minecraft server settings by selecting your Instance and clicking on Manage:
Once there, select Configuration from the left side menu:
In the Minecraft section, you will be able to edit settings such as:
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Difficulty
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Game Mode
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Spawn Monsters
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Spawn NPC
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Server Type (Official, CraftBukkit, Spigot, Paper, Forge, and many more)
After changing any of these settings, make sure to restart your server.