When I started using Scrivener a few weeks ago, I added chapters as scenes by mistake. Now I want to make them separate chapters. How do I do it?
It depends on the project template you are using. Scrivener is a very flexible platform and every project can be customized in many ways.
The base unit of text they call a "scrivening", and this is what it is--a chunk of text. They can be organized into folders, just like files.
I am using the Novel/Manuscript template which came with my version of Scrivener at the time I bought it a few years back. In my template, a scene is a base unit, and everything is built around that: scenes are organized into chapters (folders), chapters are organized into parts (also folders), and parts are just that--parts of the novel, which is essentially one huge folder, where everything is kept. It is usually called Draft or, in my version, for some reason, Manuscript.
When it comes to scrivening/file management, Scrivener mimics any and all operating systems there are: in the Binder you can move your pieces in and out of folders, create new folders/texts, copy and paste them, etc.
Compare the structure of the chapter in your template to the structure of a scene and remodel accordingly. It might be as simple as selecting the chapter you want to turn into a scene in the Binder, right-clicking on it and selecting "convert to file" option.
Quick and Dirty Way: Duplicate a blank chapter in your template which contains a few blank scenes, then make as many scenes inside of it as you need, copy-paste your text into those scenes, then move them where they belong--different chapter. I would keep the blank Chapter with blank scenes in it in your book/draft/manuscript folder just for that purpose. This is what I do: when I start a new chapter, I just duplicate the blank one, then write scenes within it.