Pry the upper and the lower part of the upper housing apart using a plastic pry tool. Do so in a motion that pushes the upper half inwards (towards the electronics), so as to unlatch the little plastic latches (see next photo) before pulling the halves apart.

There is one latch towards the front (lens side) of the flash on either side. After loosening both the upper part of the housing can be removed.

Remove the upper half of the cover completely.