How to focus a picture after its taken?

So last night me and my friends took a really cute picture, but it didn't focus so it's all blurry. Is there anyway I can focus it now? It's on an iPhone 4s

You can do a bit with software editing programs, but they won't be perfect. Photoshop is the best, but there are others out there that are free- I like GIMP. You can choose sharpening tools and sharpen all the edges. It won't be perfect, but it will improve.

Sorry. No can do. The things that you see on those CSI dramas are impossible in real life. You must get the focus and other technical aspects right at the moment that you take the picture.

The simple answer is that you can't - much.
Sharpening software effects work on contrasts and lines make the edges stand out more. If your picture has a smooth blur with noses on faces barely standing out and eyes very light, then all it will do is make it look like someone went over it with a dark pencil and did selective darkening. If the information isn't there you can't crisp it up, no matter how much NCIS or CSI have raised our expectations.

You can't really

It's impossible most of the time. And it depends on the kind of blurring - out of focus images are more or less impossible to sharpen. Images with slight motion blur caused by camera shake can sometimes be fixed in Photoshop CC with it's new camera shake filter. But even so it can't work miracles. And there's a limit as to how much you can sharpen an image depending on how blurry it is to begin with.

As of now, you can't.

However, big labs like Adobe and IBM are working with supercomputers to develop algorithms that may, in the future, be able to sharpen out-of-focus pictures. Do not delete it. Perhaps your grandchildren will have the technology to sharpen it in the coming years.

Nope… Shot is ruined. Take it again.

Only if it was taken with Lytro lightfield camera- it's possible to refocus after shooting with these cameras.
http://www.lytro.com/camera/

But with an ordinary camera, it is not possible.