Netspectrum

I have been trying through the GUI and the terminal and I am not able.

For the terminal I used the following command:

sudo gsettings set org.gnome.desktop.background picture-uri file:////usr/share/backgrounds/ubuntu-default-greyscale-wallpaper.png

What version of Ubuntu Desktop are you using?

Most people are using Ubuntu 18.04 or Ubuntu 20.04. You can easily change background via Settings Manager. See more details at https://xubuntu.org/news/small-details-wallpapers/

Or you can simply right-click on the desktop and select "Desktop Settings..."

I am using Ubuntu 20.04. It worked with right clicking. Thanks