A simple way of doing it is to make a rough selection of the sky with the lasso tool, don't worry about going over the tree tops but keep it tight on the horizon.
Go to image-adjustments-selective colour and choose whites from the drop-down menu. Tick absolute. Move the black slider up a bit till you get a grey sky, then move the cyan slider up and yellow down ... and generally play around til you get the blue you require.
If your not happy with the resulting blue... go to hue/saturation section and select cyan from the dropdown and then have a play with the hue/saturation/brightness sliders.
cheers,
Andy
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