If something is unchanged, it's exactly the same as it was earlier. If your grandmother's home is unchanged, it still has the same paisley curtains and orange shag carpeting she purchased in the 1970s.
When you visit an old friend you haven't seen in years and find him a foot taller and sporting a new beard, you might say, "You've changed!" If, however, your friend looks and acts in exactly the same way he did in the past, he's unchanged. Because many things change constantly, it can be a pleasant surprise to find unchanged things, from landscapes to people to your dad's famous apple pie.