Select Start > Settings > Personalization > Background. In the list next to Personalize your background, select Picture, Solid color, Slideshow, or Windows Spotlight (to see a new image from around the world every day).

Select Start, then select Settings > Personalization > Themes. Choose from a default theme or select Get more themes from Microsoft Store if you want to download new themes with desktop backgrounds that feature cute critters, breathtaking landscapes, and other smile-inducing options.