Step 1: Find your World Folder
1. Press ⊞ Win + R on your keyboard.
2. Type %AppData%
3. Navigate to .minecraft\saves
4. Locate the folder with the name of your world.
Step 2: Upload the World
1. Open the File Manager
2. Navigate to /minecraft/
3. Drag the world's folder into the /minecraft/ folder.
4. Change the Custom Save Name to match the world's folder name.
5. Restart the server and the save will load onto the server.