Find your world
Terraria keeps worlds in Documents\My Games\Terraria\Worlds, as .wld files named after each world. tModLoader keeps its own in Documents\My Games\Terraria\tModLoader\Worlds, each with a .twld file beside the .wld that holds the modded data.
A world saved to Steam Cloud isn't in that folder. In Terraria's world list, press the cloud icon on the world to move it off the cloud, and its file appears there.
Upload it
- Rename your copy. The server always opens world.wld, so rename a copy of your world to world.wld (and a tModLoader world's .twld to world.twld).
- Stop the server. Press Stop in your dashboard.
- Upload the file. Open Files & SFTP, choose worlds/ as the upload folder and press Upload to worlds. Files up to 96 MB upload in the browser; SFTP works too.
- Wait for the safety scan. Press Refresh until the file shows Passed safety scan.
- Start the server. Press Start. The server opens your world.
Pick the same server software
In Settings, choose the software the world was played on: Vanilla for a normal world, TShock for vanilla with admin tools and plugins, or tModLoader for a modded world.
For tModLoader, upload the world's mods (.tmod files) to tmodloader/Mods. Every player needs tModLoader and the same mods. TShock plugins go in plugins/.