Hi there, and welcome to the forums.
Basically, the first thing to say is that this kind of thing is best to get right "in camera", but I know that's not very helpful!
The solution is slightly dependent on how underexposed the forground is. If it's basically silhouetted, then there's not much you can do - there's no detail to bring back. If however it's just a little. there are options. These are all Photoshop-based answers, assuming you're using that.
Firstly, try the "dodge" tool. This is found in the main tools palette and its function is to up the exposure. Basically you set it to a relevant brush size and with "exposure" set low (around 5-8%), just brush over the areas you need to lighten. Go easy with this or it'll look odd (hence the low exposure setting), and make sure the brush is set to a soft edge.
Alternatively, you can try isolating the foreground by selecting it (
http://www.elated.com/tutorials/graphics/photoshop/selecting/) and then creating a new "adjustment layer" for "Levels". Play around with the sliders until the desired lightening is achieved.
Going back to "in camera" approaches again, another great technique is to have the camera on a tripod and do two exposures at the time - one for the sky, and another for the foreground. You can then blend the two in Photoshop afterwards. This only works if your foreground isn't moving though!
Simon
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