You could do something like this:
$("span, p").each(function() {
var text = $(this).text();
text = text.replace("lollypops", "marshmellows");
$(this).text(text);
});
It will be better to mark all tags with text that needs to be examined with a suitable class name.
Also, this may have performance issues. jQuery or javascript in general aren't really suitable for this kind of operations. You are better off doing it server side.