You can download your world on the "worlds" page: https://aternos.org/worlds/
Just select a world and click on "Download". After that, a zip archive with your world will be automatically downloaded from your server.
Make sure to use your regular browser for the download and don't use any third-party tools or download managers.
If you use Spigot or Paper you have to merge your Nether/End into the main world. This article explains how to do this: Merge the Nether and The End into one world folder