To change the Minecraft world seed on your server:
1. Go onto your server's control panel and stop your server by clicking the Stop button.
2. Navigate to ”File Manger” page.
3. On the file manager locate the "server.properties" file and click to open it.
4. Locate the ”level-seed” setting and set this to the world seed you wish to use. For example, if you would like to set your world seed to “12345”, then that line would read ”level-seed=12345”.
5. Save your changes by clicking the ”Save” button.
6. A new world will now be required to generate the new level-seed. To do this, there are two options. Resetting the current world, or creating a new world.
a. To delete/reset your current world, in File Access right click the world folder that has the same name as your current world. Afterward, click the Delete button on the pop-up model to remove that world.
b. To create a new world without deleting your current world, locate the World and rename it to another name.
7. If you haven't already, return to your control panel dashboard by clicking the "Console" button.
8. Click the Start button to start up your server. Once your server is done starting up, it should now be running a new world with the seed you set.