How to upload your own world to your Minecraft server

An existing world, yours or a downloaded map, replaces the server’s world folder. The server does not care where it came from as long as the folder name matches.

Through the file manager

Zip the world folder on your PC. The zip should contain level.dat at its top level or inside one folder.
Stop the server.
Files: delete or rename the current world folder, then Upload files with the zip.
Unzip it. If the world came out under another name (a map called Skyblock-v3, say), Rename the folder to world.
Paper, Purpur and Spigot keep the nether and end in world_nether and world_the_end; delete the old ones so they regenerate for the new world.
Start.

Big worlds: SFTP

Worlds over a few hundred megabytes go faster over SFTP. The host, port and login are under Settings, General, SFTP Access; any SFTP client works. Upload into the server folder, then rename as above.

Downloaded maps

Resources, Worlds lists maps from the platforms and installs one in a click, into the right folder name.

From another host

If the world is on a server elsewhere, do not download and upload it: More for this server, Import from another host pulls the world, plugins and mods straight over SFTP or FTP. See Move from another host.

Good to know

Match the Minecraft version. A world last played on 1.21 opens on 1.21 or newer, not on 1.20.
A world from a modded server needs the same mods installed or its modded blocks turn to air.
Bedrock worlds go into the worlds folder and are not compatible with Java servers.