Depending on the condition of the thermal paste and putty, the board may be bonded to the heatsink assembly. If you can't easily remove the board, use a spudger to gently lift the edges of the board to separate it.

Lift the motherboard straight up and off of the heatsink to remove it.

During reassembly, carefully place the board onto the heatsink with the APU facing down. Remember, there's exposed thermal paste on the bottom of the board, so use the alignment pegs to orient it correctly. If you have to reposition the board, it may smear the thermal paste.