In game, make a save. Cheat saveworld. Log off.
Download and install Filezilla.
Go to Nitrado Mods and write down your modids, and remember the order they are in.
Stop your server.
Connect to your server with Filezilla. You need 4 things for this: host, username, password, port. All 4 can be found in the FTP Credentials at the bottom of your Nitrado Dashboard. Copy and paste them, then hit QuickConnect.
Once you connect in Filezilla, your PC is on the left and the server is on the right. On the left, navigate to the folder where you want to put your backup.
Copy the entire arksa folder over to a location on your PC. You should clearly see this folder in the server view - should be the only one. Just drag and drop and let the files move.
Once it's done copying over, disconnect from the FTP (9th icon - disconnect from current server), go to your Nitrado panel and click on the Re-install option on the left panel (2nd button from bottom). Confirm that yes you want to do this - it's ok, you have a backup of everything on your PC now.
Once the server is completely reinstalled, start it, then bring it back down.
Download all your mods in the same order you had them. If you still had Winter Wonderland installed and disabled, make sure you check that box again to disable when reinstalling your mods.