Click the Apple menu in the top-left corner
Select System Settings
Click Wallpaper
Choose a wallpaper from the available options
Click Add Photo or Browse Photos if you want to use your own image
Select the image you want
Close System Settings
Right-click on the desktop and choose Change Desktop Background if available
Select the wallpaper you want
Adjust the wallpaper fit or style if needed
Wait for the wallpaper to apply automatically