Let's imagine for this example the track you want to upload it's called: My_Awesome_Track_145 (the folder name needs to be exactly the same from the one you downloaded)
- Stop server
- Go to FTP button, inside folder \content\tracks\ create a folder with your track name My_Awesome_Track_145 then inside create another folder called data, the folder structure should look like this: \content\tracks\My_Awesome_Track_145\data\
- Upload the surfaces.ini from your track to inside the data folder you created, the structure should look like this: \content\tracks\My_Awesome_Track_145\data\surfaces.ini
- Edit the server_cfg.ini file and adjust your track line with your track name, example: TRACK=My_Awesome_Track_145
- Start server
You also need to place the same track in your Assetto Corsa installation on your computer.