In general, I’m not a big fan of changing the backgrounds in photos. In most cases, it is a rather complex undertaking and I honestly don’t believe it is worth the time or effort. But, with that being said, there is an exception to every rule, even this one.
Start with a green or blue screen
As opposed to attempting to work with a busy background, staring out with either a green (or the less common blue) screen will make things a lot more manageable. While both of these tools are specifically designed to allow for background adjustments, you should still be careful to avoid subjects with colors that are very close to those of the screen itself.
A couple against a basic green screen.
Using Adobe Photoshop (if you use another editing application, you will need to determine if you have a similar tool available), select the magic eraser tool and click anywhere in the background to remove it. Note that on some occasions, you may have to clean up any background color that does not get erased on the first attempt.
The magic eraser icon.
After removing the background.
Last but not least, open the photo you want to use as your new background. Simply drag your adjusted image over onto your new background and there you have it!