With this complex picture with all the trees branches it would take a long time to make a mask for the sky. Here is a simple step to how to do this fast BUT, it does not always work on every image thats why you need to know how to use different methods.
Ok
1) get your magic wand tool and select the sky.
2) go to the select menu and choose similar
3) get your brush tool and choose color in the drop down menu of MODE
4) choose a blue you want.
5) adjust the opacity of the brush to like 50%
6) paint the the sky
7) if there is any spots that was not selected the first time now go back and repeat.
I did this in about 1 minute.
Now my CC for the image
I like the wagon it shows character but It would be cool if you got more perspective on it and offset if from the center. Needs more black and if you know how to create a selective color mask and add more color I think you could make this a very nice picture. I just feel it needs more,....but, my opinion.