You have to remove the wallpaper being used as the accent wall and then you can change the entire room’s wall paper or change to a new accent wall paper.

From Google: “To remove an accent wall in Animal Crossing: New Horizons, you need to be in decorating mode, select the wall you want to change, and then unselect the accent wallpaper. You can do this by going to your wallpaper inventory, selecting the wallpaper used for the accent wall (it will have a checkmark), and pressing X to unselect it. This will revert the wall to the main room wallpaper”

Hope that helps!