When you click on the Import Website link from the "Websites" Actions menu, a file chooser popup will be provided to select an appropriate
.dat file to be used for the import.
After the file has been uploaded/imported, it will be listed on the All Websites page with (restored) in the comments.
You can then make the necessary changes to the site.
The imported site does not replace the current "default" site.
To set the new site as the "default" see Use Multiple Websites.