How to change the difficulty of a terraria server

To change your world difficulty, you will need to create a new world since the one that has been generated used the default difficulty. Follow these instructions :

1. Stop your server
2. Change your world name in the "settings" page
3. Change the difficulty in the "settings" page
4. Go back to the manage page and click "restart"

If you don't want to generate a new world, you can still use a tool called TEdit to change your world difficulty.
Once the difficulty changed, you can set the right difficulty of your Terraria server and upload your modified world to the server.