You can set the server difficulty in a couple of ways.
The simplest way would be to log into your MultiCraft control panel, click on Console on the left hand side and run the command "difficulty [desired difficulty]" to change the difficulty immediately between Peaceful, Easy, Normal and Hard. Alternatively if you are opped on the server (or have permission for the command if you are using plugins) you can use the command ingame as well, with a / infront of it as normal.
You can also edit the server.properties itself for this on the Files > Config Files > Server Settings page of the control panel. Simply change the Difficulty setting to what you want, save changes at the bottom of the page and then restart your server.
**Please note that there is currently a visual bug on MultiCraft for Minecraft 1.14 and newer servers that shows the setting on the Server Settings page as Peaceful all the time, regardless of the real setting - this is being worked on but for now you can confirm the real setting in the console by just typing in "difficulty".