1. Firstly, minimise or close all your applications so you can see your background. You can do this by pressing the dash or the 'x' in the top right of the application to close it. You can also use the Windows command "Windows Button + D" to view your desktop.
2. Once you can see your background right click (Press the right mouse button) anywhere on it to bring up the context menu.
3. Navigate to the "Personalize" option on the menu and press on it. This should bring you too the personalization tab of settings.
4. Look for the background tab, it should be located near the top.
5. From the background tab you can select from Microsoft's own default backgrounds or by pressing on "Choose a photo" you can pick a picture from the computer to use as a background. You can also control the fit of the photo and if you want a slideshow background or a solid colour background.
6. Once you've selected your background it should save automatically, there is no apply option just return to the desktop to see if your background has applied.