In order to generate a new world you will simply need to go into your "Configuration files" section of the control-panel and edit the "Serverconfig.txt" using the configuration editor.
You will see a field labelled "World:" and the name of your world (default is world1). All you have to do is change this to something else and save the file, when you restart the server it will create a new world for you. If you wish to name the new world something specific, you change the "World Name" to what you want in the same configuration editor.
If you have any questions in regards to this, don't hesitate to open up a support ticket and we'll help you out!