In a Windows Forms application, you can update the background color of a control, such as a form or a panel, by setting the `BackColor` property of that control. Here's a step-by-step guide on how to update the background color in a Windows Forms app:

1. Create or open your Windows Forms application in Visual Studio.

2. Open the form or control for which you want to change the background color in the Visual Studio designer.

3. Select the form or control by clicking on it in the designer.

4. In the Properties window, locate the `BackColor` property, which controls the background color.

5. Click on the value field next to the `BackColor` property to open the color picker dialog.

6. In the color picker dialog, you can choose a predefined color, select a custom color, or specify the color using its RGB values.

7. After selecting the desired color, click "OK" to close the color picker dialog.

8. The `BackColor` property will now be updated with the selected color. You can see the background color change in the Visual Studio designer.

9. If you want to change the background color programmatically (e.g., in response to a button click event), you can use code like this:

myControl.BackColor = Color.Blue; 

  Replace `myControl` with the name of the form or control you want to change, and `Color.Blue` with the desired color.

10. Build and run your Windows Forms application to see the updated background color in action.

Remember to replace `myControl` with the actual name of the form or control you want to change, and choose the background color that suits your application's design.