When you set up your first Minecraft server, it starts off with a freshly generated world. In case you already spent a lot of time building your own world and want to import it to your server, you can do that easily by following this procedure.
How to find your world?
Windows 10
Press the Windows Key (Start)
Type in %appdata% and press enter.
Open the .minecraft folder and then enter the saves folder.
Find your save and drag the folder to your desktop so you could find it easily.
Rename the world so you can identify it later. You can use any name you'd like.
Mac OS
Click on the Spotlight button in the menu bar, it looks like a magnifying glass. Or, use the Command+Space keyboard shortcut.
Paste ~/Library/Application Support/minecraft into the search.
Open the saves folder and drag the world to your desktop.
Rename the world to something that you can remember for later.
How to upload your world?
In order to upload your world, firstly log in to your Game Panel, select your Instance, click on Manage:
Choose File Manager:
Once there delete the current world folder by using right-click and then Trash (Make sure that your server is stopped before doing this):
The next step is archiving or compressing your world folder so you could upload it:
Once that’s done, drag your world archived/compressed file from your desktop to the File Manager window:
Once your world zip file is uploaded, you'll need to extract it by using right-click and then Extract:
That’s it your world is now uploaded!
The last step you should take is to simply log in to your server and check to see if the map was changed correctly. If it's not changed still, make sure that your map name is using small letters only.