Once you've removed all the screws from the metal holder, carefully lift and pull all the top elements of the keyboard backwards together, leaving the white key holder on the floor.

Gently place the white key holder to one side, don't tip it or you could lose some of the key nodules.

Place the metal holder back into the keyboard retainer as in picture 2. Then gently fold the membrane back into place.

At this stage if you can see any liquid residue or dirt on the membrane use a very lightly damp cloth to clean, or a some baby wipes.

Do Not use lighter fluid, turpentine, or nail polish remover. Anything strong like this could destroy the membrane.

Once you think you've got any dirt or residue you can off the PCB membrane, let it dry and move onto fixing the keys.