The picture doesn’t change, but your brain interprets it in two different ways.

When you look at something, your eye and brain must decide what it is in a  split second. Your brain makes a ‘best guess’ and usually gets it right.

But this picture is ambiguous. Your brain can’t decide which is the best ‘best guess’, so the picture appears to flip between the two options.

Whether you see the old or young woman partly depends on where you look. Looking at the eye / ear provides strong evidence for the old woman, whereas looking more to the left of the picture provides strong evidence for the young woman, so that is who you see.