Select Filter > Noise > Dust & Scratches… from the top menu bar. Start by putting your Threshold slider to 0, so it can fully analyze all the pixels in your image.
Put your Radius slider at 1, and increase it by a single step until you see the dandruff disappear. You might notice that your image gets a little soft as you do this. I usually keep mine below 4.
Finally, increase the Threshold slider until you get the desired effect. In the below example, I have it set to 39 levels.
If you have some larger specks or other imperfections, simply use the Spot Healing Brush to remove them.
Note that you may experience some loss in detail with this technique. If you have high-detail elements to your photograph, I recommend creating a duplicate layer before the above steps, and masking the high-detail areas.