I assume (like Tom) that you mean the band of light area I have circled?
If so then as always there are a variety of ways to address this.
EG
Duplicate the layer
Make a selection of the sky - I used select by color range)
Promote the selection to a layer on its own (using Ctrl + J)
Create a new layer above this
On the new layer create a gradient fill using dark blue to a slightly lighter blue
Now clip the gradient to the layer containing the existing sky (to do this hold down Alt while you SLOWLY move the cursor between the 2 layers - the cursor will change to a bent arrow with a white box, at this point left click)
You can vary the intensity using the Opacity.
Another way is to completely change the sky -
Again select the sky (using your preferred method)
Create a layer mask while the sky is selected
Place a new sky layer below that layer.
There must be at least a dozen more ways of doing this!!
Cheers
John
OH Welcome to the forum - and I really like it when people ask HOW to do something - that way we all learn something!!