How you go about downloading a background depends on the OS (Windows or Mac OS).
Also, be aware of copyrights; if you intend to use this background and image professionally, be sure you have permission to use them if they're not yours.
Simple way of doing this:
Open the background. Open the image you want in the foreground. Select > All on the menu bar. Click and drag the contents to the background image by dragging to the background image's title tab. Release.
Do a search for tutorials on replacing backgrounds using Photoshop. There are many to be found. Look at the one that use masking to extract what you want in front of your new background. There are many ways to create mask. The new background is the easy part of the job. Removing the old background is the complex part of the job.