Bring your singleplayer world or an old server's world with you. All you need is a ZIP.
Get the world ready
Find the world folder on your computer. For singleplayer, it is in the saves folder of your Minecraft folder. The right folder has a level.dat file inside it.
Zip that folder.
Upload it
Stop your server.
Open World tools in your server panel.
Under Upload a world, choose your ZIP.
Pick Replace world to swap out the current one, or Keep current world to upload it to a new folder and switch to it.
Press Upload world, then start your server.
Your old world is safe
Kixa takes a safety backup first and keeps it for 24 hours. You can find it under Backups.
Common questions
Can I download my world too?
Yes. Open World tools and use Download world to get a copy of it.
Why can't I press Upload world?
The server has to be stopped first. Stop it and try again.