The power supply is connected to the bottom of the main board assembly by metal prongs that plug into the bottom left corner.

Insert the flat end of a spudger between the board and the lower shield plate, by the two parallel solder joints near the corner with the power button.

Use your spudger to pry up the board until the two prongs come completely out of their socket.

You should feel the prongs slide out.

During reassembly, press down on the corner of the board to fully seat the prongs in the power supply—you should feel them slide into place.