There is a lever on the inside of the backbox on the top, slide that around until you can gently lower the mech panel with all the score reels etc on it.

Then from the side, put one arm around to the front of the glass and another arm around to the back.

Now place your hands on either side of the backglass

Push gently towards the back of the game and the glass will be free, now there is a channel at the base of the backglass in the timber.

Place the backglass in this channel and from here you will be able to lift the glass out from here.

Just be careful of the lever on the way out.

Rob, if in doubt just bring the entire head with the glass in it