How to put a personal world on your Minecraft server?
In case we are working on a world named world by default.
Stop your Minecraft server.
Rename the new world to put in world.
Connect in FTP on the server with an FTP client like FileZilla (tutorial) (or with WebFTP from the OMGSERV panel but this is not recommended because many bugs).
Delete the existing world folder.
Transfer the new folder to the FTP root.
Be careful ! This deletes the existing map, to keep it, make a backup beforehand.