This picture is quite difficult for changing the background because:
- Too small. Need higher resolution to be able to treat the edges for a more natural effect.
- Not enough contrast between the subject and the background. For example, the hair of the 2 adults and the night sky in the background, the left arm sleeve of the man ad the wall.
Anyway, I have given it a try and here's the steps and results:
1. In PS, open a new file the same size as the image. Select a sky-blue as background and white as foregound colours in the colour picker and the use Filter>Render>Clouds to generate the following to be used as the background:
2. Paste the original image onto a second layer on top of the rendered background. Use tha Marquee tool to do a general selection of the subjects. Invert the selection and delete:
3. Carefully erase the remaining background using the eraser tool. Need to use various brush sizes, strength and feathering to make the edges look natural. For this image, I had to use some blurring on the edges also:
4. Finally, adjust the levels on both the background and subject layers so that they are more compatible. I also added a drop shadow effect, hopefully making it more natural. To be realistic is quite out of the question for this picture. So here's the final result: