Sometimes you might want to use Classic Pages in SharePoint online, but there is no “use classic pages” link in the lower left hand corner. What to do? I don’t know why the “Switch to Classic Pages” link is not there, but here’s a way to fix it.
Open Site Contents
Open a browser console (usually Ctrl+Shift+i)
Put this in the console
document.cookie = “splnu=0;domain=” + window.location.hostname + “;”; location.href = _spPageContextInfo.webServerRelativeUrl + “/_layouts/15/viewlsts.aspx”
Switch back to the browser; you should be in classic mode.
To get back to Modern Pages, re-run the same line in the console, but switch splnu=0 to splnu=1
Learning SharePoint 