Every time you save a page, a version is stored automatically — per language, with date and user. You do not need to do anything for this.
Open the history
In the pages overview, open the options menu of the page and click View History. You see a list of all saved versions with date and user. If your website has several languages, pick the language at the top.
View an old version
Click View on a version. It opens in the editor with the live preview — just like editing, but read-only. You cannot save or change anything here.
Get old content back
Want something back from an old version? Copy it. Copy all sections takes the whole page — or copy just the single section you need. Then go to the current page and paste it there — see Copy & paste sections.