If you haven’t already, turn on the Wallpaper app from the Apps panel. Instructions can be found here. Then, click the “Wallpaper" app button on the OneLaunch dock.
To change your desktop's wallpaper, simply click on a wallpaper from the displayed choices. To view more wallpapers, click the arrow button located to the right of the shown wallpapers.
To narrow your search for your next desktop background, you may select a category from the drop-down menu on the left-hand side to find wallpapers within that category. Alternatively, you may enter keywords into the search box and press "Enter" on your keyboard.
If you would like to revert changes you've made to your desktop wallpaper, click the button to go back to your previous wallpaper.