You can't make it perfect, but there's an algorithm with the fancy name "Richardson–Lucy deconvolution" which can get impressive results.

The open source and free software program G'Mic has a feature to do this. It's implemented as a plugin for GIMP. You can find it in the "Details" section of the G'MIC plugin, which is under the Filters menu.

Play around with the sliders. "Sigma" is the most important; it basically corresponds to how big the blur circles are. Somewhere around 4 seemed to work for this image. I ran it with 100 iterations; this makes more speckling but has a stronger sharpening effect, too.

The image is also significantly unfderexposed — probably the camera was fooled by those bright lights in the backround — so I used a quick shadows & highlight adjustment too. You could also play around with those manually to get things better, but this is a quick fix. The resulting image has some speckling and other artifacts; I think for a final result you'd want to do some by-hand touchup. But this should get you started.