The batteries are not replaceable I am afraid. They are an NIMH 800(mah) 1.2 v battery.
The video I do believe is correct.
The Solar panel is on the top, so we would not recommend them to be mounted this way as they may not get the sun they need to charge.
The batteries cannot be replaced.
The decking lights are screwed down to the decking on the outside of the lights, so will not be fitted into the decking, so be mindful of fitting these onto stairs.