If you want to keep the existing world for later, first make a backup of your world. Backups are explained in this KB article.
Before uploading your world, you'll need to know the name of your Virtual Gladiators server world. To do this, login to your Multicraft control panel and look at the "World" value:
In the above example, the world is blank, which means it's the default name of "world".
If the world you are uploading is also named "world", you'll need to delete all the world directories on your server before uploading the new world. You can do this through FTP, which is explained in this KB Article.
If the world you are uploading is named something else, such as "Cool World 2013", you'll need to enter this name EXACTLY as it appears in your multicraft control panel. It is case-sensitive, so make sure it is exact:
Don't forget to click the save button at the bottom.
Now just use your FTP client to drag and drop the world directories to your server.
When the upload is complete, start your server and your new world will be available.