You can click on the arrows to the left and right of the number (here marked red), and that will change the number by exactly one each click.
So if you moved the box with the number to the "12" as shown here with the mouse, but want to bid 10, you click twice on the left facing arrrow and the number will decrease twice to 10.
So if you moved the box with the number to the "12" as shown here with the mouse, but want to bid 10, you click twice on the left facing arrrow and the number will decrease twice to 10.