Click "Eidt" on the top toolbar and you can edit the bitmap. There are two tools used to edit your bitmap.

You can select Magic wand to delete an area from the image. The parameter Fuzziness works with the magic wand, indicating the range of pixel colors you delete at a time. The larger the value, the wider the color range, that is, the colors of the pixels to be deleted can be quite different; the smaller the value, the narrower the color range, that is, the colors of the pixels to be deleted are very similar.

You can select Eraser to erase a part from the image. The parameter Size works with the erase, including the size of the eraser. The larger the value, the larger the area you erase at a click.