There are two ways to do this, and the in-game one is far easier. Only use the panel if the upload keeps failing.
The easy way
In Server Manager, select your server, open the Save Games tab and click Upload Save. Pick your .sav and the game does the rest.
Through the panel
- Stop the server.
- Find your save on your PC in
%localappdata%\FactoryGame\Saved\SaveGames. - In Files, go to
FactoryGame/Saved/SaveGames/server/and upload the.sav. - Start the server, then load the save from Server Manager in-game.