Remove two screws securing the motherboard.

On reassembly, note that these two screws (only) have triangular indicator marks against them. The other holes are for screws which you removed from the bottom of the laptop.

You should now be able to lift out the motherboard out of the shell, gently easing the connectors on the left hand edge out of the shell.

As you lift out the motherboard it's a good idea to lean the screen against some kind of support as it will no longer be counter-balanced by the base.