Okay, you need to practice using layers ....... and to get used to what the "select" an image can do.
1. On one layer, have your scanned image.
2. Draw a rounded rectangle on a new layer: rectangle shape found in the shapes area, under basic shapes.
3. With the magic wand, select the inside of the rectangle. Check flood mode (in the upper bar on the canvas) is set to 'contiguous'.
4. Move to the image layer and use Ctrl + I on the keyboard, which will invert the selection.
5. Then hit Crop to selection - or just hit delete.
6. Uncheck the rectangle layer and flatten.
7. Use AA's Assistant to clean up.
But, remember that saving as .png will retain transparency.