Those are captive screws and cannot be removed. The service manual also instructs you to use a plastic scribe to loosen the retaining clips around the perimeter of the cover. The support page can be found here. This video should help as well.
I am trying to replace the battery.  The first step is to remove the laptop back cover.  I loosened the cover 5 screws, but I was not able to remove the screws.  After watching a few videos online, I came to the conclusion that perhaps it is not necessary to remove the screws and it is sufficient to loosen them up.   I tried to pry the cover using a plastic tool with the screws loosened (but still in the cover) and  I was able to insert the tool under the cover just at the corners of the laptop and that was not sufficient to pop the cover off. Could somebody kindly let me know the following: Please note that this is not a problem of wrong screwdriver or stripped screws.  The screws turn normally in both directions and I can tighten them back if I want to. Thanks  
13 R3, back cover does not come off