To change the seed you have to edit your server configuration file.
1. On your Multicraft panel on the left menu go to Files -> Config Files and click on Server settings.
2. Scroll down to the line where it says “Level Seed” and type your seed into the box.
3. Go back to your main server page and in the “World” text box type a name for your new world.
4. Save and restart the server and the server will make your new world with the new seed.