Notice the histogram, under it there are three tiny arrows, one on the left, one center and one on the right. These arrows are used to adjust the "levels" in the image. You can manually shift them back and forth by using the cursor to select one, hold down the mouse left click and sliding it where you want to go. First, let's adjust the "white" setting.
In our picture white will be easy, examine the non-necklace area, all of it is white, find the area that looks lightest. Now, using the mouse cursor, select the left hand arrow and slide it all the way to the right. Next, use the cursor to select the right hand eye-dropper. Move the eye-dropper to the area you decided was most "white" and then click the left hand mouse button. Just before I clicked on the left hand mouse button I captured Figure 5.
Figure 5: Left slider all the way right, eye-dropper is in mid lower-left quadrant