There is currently the option for four world generation types: DEFAULT, AMPLIFIED, FLAT or LARGEBIOME. 

1. On the left hand side of your control panel click "Config Files".Choose the first file named "Server Settings"
2. Scroll down to the level-type field.
3. There are 4 types of world to choose from DEFAULT, AMPLIFIED, FLAT or LARGEBIOME
4. Choose a world type and enter it into the box and click "Save"
5. Delete your current world or create a new one. (See here)
6. Start your Minecraft server.
   

Please see here for an explaination of each world type.