See the two sliding arms at the top - pull them towards you (facing the rear of the head) - this creates a "cavity" for the glass to GENTLY move back towards you. Now the glass can be lifted (from the front) into that cavity. Then pull the glass out (by the channel).

There might be a small label stapled to the LHS of the headbox with a brief warning about removing the glass.

From memory - this is how they are removed - Remove GENTLY - and with two people - just to be safe.