There are a few different ways to change your desktop background. If you have a picture open that you would like to use, simply right click the picture. A menu will pop up, select Set as Wallpaper.
Another option is to right-click directly on the desktop or by going to Settings->Personal->Background. From the menu, select Change Background. A dialog box will open and you should click on the box that says Background. That will open a separate dialog box, where you can choose a new image to set as your wallpaper. Move between the options at the top to find the image you’re looking for. You can choose between one of the pre-loaded wallpaper options, one of your own pictures, or a color. Double click the image you would like to use and your desktop background will update instantly.
Finally, you can change your background in Settings.Click the Endless logo icon at the bottom left hand of the screen.
The Endless logo icon looks like this: