Getting the backglass out of an EM head

Steps

Open the metal back door of the head.
Where the head has a pair of slide latches along its top edge, push them rearward; the glass can then be raised and withdrawn toward the player using the lip at its bottom.
On many Gottlieb heads, release the latch so the mechanism board swings down from the top on its hinges, giving room to lift the glass out.
If the board is screwed in place (four or six screws), remove the screws and tilt the board back to reach the glass.

Source: Pinball Machine Care & Maintenance, Bernard B. Kamoroff (Bell Springs Pinball Alley)