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 on the Delete button at the top.

5

Fifth Step

After that, Head over to General tab, Click on Console and Start the server.

There you go! You have reset your server’s worlds!
How to Download your WorldHow to Change Server Versions