Log into your server's files via an FTP program such as WinSCP or FileZilla.
Look for the folder named "world". Your server may have one world folder for all dimensions or multiple folders for each dimension depending on the version your server is running on.
Find the world file you would like to upload. If your world is in a normal folder, just upload the folder. If the world file is in a compressed format, upload the file then extract it within the panel.
Move the world file into the main directory (this is the area your world was previously located).
Once the world has finished uploading and has been extracted, rename the folder to "world" by right clicking the file and clicking "rename".