Changing the background in Joomla is relatively easy, and can be done by following these steps:
1. Open the administrator panel and go to Extensions > Joomla > Backgrounds.
2. On the Backgrounds screen, you will see a list of available background styles.
3. Select the style you want to use and click the Edit button.
4. On the Edit Background screen, you will be able to change the background image, color, and transparency.
5. Click the Save button to save your changes.