OK, just went to the site, downloaded the tgz file for the Autumn Castle. I went to "My Worlds", Clicked Manage Worlds, then was taken to the manage world pop up. I clicked The File Tab. Under Replace World, I clicked "Select File" where it says upload oar. I selected the tgz file from my downloads folder and pushed replace world. It prompts that your present world will be erased, uploads the file when you agree and takes a couple minutes to replace the world.
It informed me when it was done, and I logged in to a very temporary new look for my world.
Easy peasy! Note, I am not trying to oversimplify it for you, this post was made with future people in mind that may run into the same question.
I hope that helps.