Are you referring to your wallpaper or how your apps are arranged on your home screen?
To change your wallpaper back to what it was: Swipe down your notification shade and tap on the gear icon on the right corner> next scroll down to "wallpaper and style" there you can change your wallpaper.
However if you wanted to change your theme back to what it was: in the same settings menu instead of going to "wallpaper and style"> tap on "themes"> then at the bottom right corner of your phone, tap on the "menu" button> then tap on "my stuff" at the top left. Which you will see a row of things you can change back from "themes" Wallpapers, AOD (Always on Display) and icons.
I did not communicate that sufficiently. Apologies. I have multiple screens, one has always been the default screen with most used apps. It randomly changed to the 3rd of 4 screens that it default to and shows the home Icon on the 3rd screen.
Hello, thank you for your feedback. I understand your concerns with wanting to change your home screen layout and I will be more than happy to help you.
If you unlock your phone, touch and hold down any empty spot on your current home screen. At the top of your pages you will see a house icon, simply swipe to the page that you would like to be number 1 and tap on the house icon. You can also tap and hold down on the pages to move them around in the order you would like.
Here is some additional information on personalizing your home screen
