You maybe want to upgrade your server to a newer version, but if you already did this, you will see that everything is cleared, the world and your inventories.
So, if you want to keep your items with you even after the upgrade action, just keep reading ( It only save players' inventories, not the world itself, so buildings will be gone, so sad, but you can also save it with other methods, like removing all the unloaded chunks and replacing them with new ones from the version you want to upgrade or downgrade to ):
First, Backup your world.
Then, click on Download button of the old world, afterwards, upgrade ( or downgrade, as your needs ) your server's version, it will give you a warning of world deletion. accept it and take the risks lol, anyway it will automatically generate you a new world.
So you've moved your server to another version, let's jump to the important part. You will have to download the new world, extract both downloads, then go to your old world files and enter "{world's name}/world", copy the "playerdata" folder and paste it in the same place in the new world folder. After all of that, impress the folder you paste the "playerdata" in, and then upload it to your server's files.
I guess everything is done, well, partly, go to "Files" in Aternos Dashboard, then "world/playerdata", you will see some jebrish with players' name, I suggest joining your server in creative mode ( to avoid suffocating after joining ), then enable "keepinventory" and kill yourself. take the coordinates of spawn and then go back to aternos, paste the coods in every player's data.
and Done!