Hey.
Just another approach.
I used the Gradient Tool set to the Foreground to Background preset. I point sampled the colors for the foreground and background from these two places.
Note the red X's
On a new layer above the original image, I then created a gradient trying to best match the angle.
I placed a layer mask on the gradient layer.
I then selected the layer mask and used the Brush Tool (set to black) and a brush set to 0% hardness, 1% opacity to blend the bottom of the Gradient layer into the existing sky background.
I do this to get the best possible blend between the new BG and the old............including the trees to the right. It's very hard to get a perfect selection that looks natural.
Here I have not yet finished masking out the tower. I will do this later.
Note how the trees and the tops of the building look more natural.
Later, I will also show how to remove the remaining tree off of the building.