Wanting to go back to way my mac was before icloud was restored.

I recently got a new Mac, around three weeks ago. Up until yesterday, nothing from my old Mac had transferred. I was still logged into my icloud account and everything, but none of my files or anything had moved to my new computer. I was initially really upset about this, but I came to realize I much preferred it. I got a fresh start, and the random files I had saved to my old desktop were gone which made things much easier. My deskstop was clean finally and I got a new start to customizing my Mac before college. Then, yesterday my Mac randomly did what I thought was an update (please bare with me I am not tech savvy at all) but when I logged on to my computer again, all my stuff from my old Mac was back. Also, apps I had downloaded were removed, and files I had downloaded were lost as well. Is there any way to revert this change? Or like, go back to the version my Mac was at before this? I really want the old one back. Please help. Thanks!