Log into Multicraft and follow these steps:
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1. Stop your server
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2. Click Files
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3. Click Config Files
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4. Click Server Settings
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5. Scroll down to the Level Seed field and enter in the seed you want
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6. Click the save button
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7. Return to the main panel page
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8. Go to the world box and enter a world name (can be anything, ie: myworld) - You need to create a new world name (otherwise, it will load your old world)
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9. Scroll down and click the save button
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10. Start your server.