Step 1: Know what visitors see
Your site went live when you signed up. Visitors see each page as it was the last time you pressed "Put live". What you change in the editor waits for you until you press it.
If the banner at the top of your admin says "Your site is not published yet", visitors see a coming-soon page instead of your whole site. Click "Publish checklist" in that banner to take it live.
Step 2: Put your changes live
In the admin menu: Website, then Site Editor
The bar at the top of the editor counts the pages you changed that visitors do not see yet. Click "Put live" and every changed page goes live together, in one go. The bar then says "Live now." with a link to see it on your site.
"Put live" is greyed out when nothing has changed, and while a change is still being kept or could not be kept.
Step 3: Undo the last Put live
In the admin menu: Website, then Site Editor
Click "Undo last Put live". It asks first, naming the pages and the time they went live. Click "Put it back" (or "Put them back") and those pages are live again exactly as they were before, their search engine title and description too.
Your changes stay in the editor, not live, so you can fix them and click "Put live" again. It undoes only your most recent Put live, and only once. With nothing to undo, the button is greyed out.
If one of those pages has gone live again since, it cannot be undone, and the editor says so without changing anything.