Direct Upload:

1

First Step

2

Second Step

Select your Server

3

Third Step

Make sure to Turn OFF the Server

4

Fourth Step

Under the General tab, Click on Files, Then select world, world_the_nether, and world_the_end and click the Delete button at the top.

5

Fifth Step

In your local storage, ZIP the world file you want to upload to the server.

6

Sixth Step

On the panel, Click the Upload button and select the zipped folder you wish to upload

7

Seventh Step

Once it has been uploaded, Click on the 3 dots next to it and Click Unarchive

8

Eighth Step

Click the 3 dots of the unarchived file and Click Rename and rename the file to world

NOTE: Capitalization does matter

There you go! Now you know how to upload your own Custom World!
How to upload a custom resource packHow to Download your World