The easiest way is if you shot RAW. Just recover the highlights and you'll get some of it back or make multiple exposures and merge them, or a combination of both. You can go back in and fix it on a jpeg, but it's a lot more work and there are many different ways to go about it (I'd start by using layers, bring down the exposure on one, mask that area off, bring in the blue...) But either with jpeg or RAW you'll still have some blown out parts unless you bring in sky from another shot, and then you'll have to work to blend it to the rest of the image.