How do you access keyboard to fix broken key without taking whole piano apart. Many videos found for keyboards but not for this piano.
You are talking about the HP-3800g, right? From looking at the service manual (which does not tell explicitly how to do this), you first have to remove the side panels, which I think are secured by two screws from the inside/underneath. In other words, poke your head under the piano, and look for two screws into the side panels - one just under the lower surface of the keyboard, and the other about a third of the way up.
When the side panels are removed, then you can remove the top board by removing screws from the back of the piano and from the sides.
It looks like you’ll then need to remove the control panel to get to the back of the keys - but the manual doesn’t give an indication of how to do that, so hopefully it is obvious!