Wikipedia:Training/core/Sandbox edits for existing articles
When you start to revise larger chunks of an existing article, it's a good idea to draft your first significant edits into a sandbox. Don't copy the whole article, just choose the small piece you want to work with. Note though, that when you do so you must provide copyright attribution in your edit summary, by stating the copying taking place and wikilinking the source of the copying. A model edit summary: "Copied content from [[page name]]; see that page's history for attribution."
Trying to rewrite an entire article in a sandbox can be annoying to editors who work on that article. Other editors will keep making good edits or expansions to that article while you are revising an old version in parallel in your sandbox; if you just copy and paste the whole article from your sandbox sometime later, you will undo all of those edits! Focus on small chunks of the text you want to edit instead.
It's polite to use the talk page of the article to link to your sandbox when you are ready to copy things over. That way editors can see what you're up to, and post suggestions to the talk page before you start. Once you are happy with your sandbox draft, you can place another notice on the talk page of the article with a link to the sandbox, explaining what you've done and asking for comments on it once you've added it to the main page.