The server should be stopped when making changes to it.
Maps can be downloaded from https://www.beamng.com/resources/
You will want to use an FTP client to upload the map's .zip file to ./Resources/Client/
Maps should be automatically downloaded by users when joining the server.
Next you will need to obtain the map name.
Open the .zip file for the map on your PC.
Look for a 'levels' folder and open it
There should be a folder with the map's name in it. Copy down the map's name.
Login to your server's control panel
Navigate to your BeamMP server.
Stop your server if it is running.
Click the Configuration Files option.
Click the Config Editor icon for the ServerConfig.toml
Edit the Map Setting to point to the map.
If the map's name was cool_canyon, the map setting should look like
/levels/cool_canyon/info.json
Click Save.
