You can easily edit your Minecraft server settings by selecting your Instance and clicking on Manage:
From there, select Configuration:
There you will be able to edit such settings as:
Difficulty
Game Mode
Spawn Monsters
Spawn NPC
Server Type (Official, CraftBukkit, Spigot, Paper, Forge, and many more)
NOTES:
Don’t forget to restart your server after making any of these changes!