Right-mouse click on the desktop, select Properties, and on the Background tab, select the pattern you want to change. (Make sure None is selected under Wallpaper, wallpapers take precedence over patterns.) Click on Edit Pattern, and use the Pattern Editor to make your changes. Click on any square of the pattern to toggle its color between black and your desktop's background color. (In other words, if your desktop's background is set to black, you won't have much luck editing patterns!) When you like the result in the Sample area, type a new Name for the pattern and click on Add. (Or, to overwrite the existing pattern, click on Change.) Click on Done to exit the Pattern Editor.

To view your pattern on the big screen, click on OK to close the Display Properties dialog box. In the future, you can select your new pattern by name in the Pattern list.

Note: If you're interested in seeing where these changes are stored, open control.ini (usually located in your \Windows folder), and look for the [Patterns] section.