need to replace rubber power button external part

Is the button (the part you touch) separating from the devices' frame? Or is the button (the interior mechanism that's activated by applying pressure on the part you touch) failing to work ie it clicks but does not always turn on/off when it does?

What you've described sounds more like the former, which is good, because the fix is way easier: In most cases the button just needs to be re-seated in the frame/button assembly. You can do this by opening the tablet up and maneuvering the exterior button back into place from the inside with a pair of tweezers.