Press on it with your thumb nail in the direction opposite of the small peep sight until it pops up. If you've never done this before and your rifle is brand new, it will be on the stiff side. It's normal though as it was meant to be by design. As you do it, the sight will get a little easier to switch back and forth, but not so much that it gets loose and switches unintentionally. Keep in mind as well that there may be some of the packing grease in the sight aperture which could also be gumming it up. Both my 16 and 17 had some in the rear sights. It was easily fixed though when I sprayed some cleaning oil in there and brushed it out with a nylon cleaning brush (same one that comes with a standard AR-15 cleaning kit). Hope this helps.
As far as adjusting the iron sights. Start on page 29 of your manual. It covers adjusting the sights and how to zero the sights. After completing the zero at 25 meters you will be able to adjust for windage and elevation.