Step 1: Download the world you want to use and save it somewhere accessible.
Step 2: Make sure the world is in a ZIP archive. You can do this by using a program like 7Zip or WinRar. When a world is not zipped it will take a longer time to upload the world folders because it has to upload all the chunk files one by one. Upload your files using SFTP.
Step 3: On the DedicatedMC.io Panel go to File Management and press upload. Here select the ZIP file of your world and upload it.
Step 4: When the ZIP file has been uploaded, click the three dots on the right of the file and press 'decompress'. This will unzip your ZIP file and place the world files in your server directory.
Step 5: Make sure that the world folder is named
world, otherwise rename it to world.
Step 6: Restart your server. Your new world is now uploaded and ready to play on!