To get to the "back" cover you have to almost completely dis-assemble the laptop, (not the lid assembly components) doing this whilst the battery is still connected to the motherboard and supplying power to it. The laptop may be "off" but the power is not isolated from the motherboard.
This is not be the safest approach to take and may not even work.
To remove the motherboard may not even be physically possible with the battery still in situ.
Here is a link to the service manual for your laptop, taken from this HP webpage
Scroll to p.89 to view the "back cover" when everything has been removed.
Here is the image taken from p.89 showing the position of the latch mechanism. As you can see everything in the base has to be removed to get at it.
(click on image to enlarge for better viewing)
You may either have to push down on the battery release latch or perhaps the battery to see if you can get the latch to slide.
Hopefully this is of some help.