Backend and published webpage don't match up

Hi, apologies for the simplicity of the request but I’m really struggling! I liked the multi-author full-width blog layout on the demo and was trying to work out how to replicate it. I imported the demo theme to see how it was done, and it’s overwritten my previous pages. However, in the backend, they are still laid out and formatted exactly as I have set them up but when I click preview (or publish and preview) my homepage is the demo page. Please help! How do I revert back to my original layout?