The picture below shows the playfield release lever. Many folks new to games often wonder where this "release lever" is that releases the metal plate that holds the glass in place. It can be found towards the right side of the game after you open the coin door.
Once the lever is pushed to the left (typically), you can remove the metal piece that holds the glass in place, slide the glass out and store it on end, and then move the lever to the left again, lift up slightly on the playfield, pull the plunger back so that you don't damage the plunger tip, and lift the playfield all the way up. Have you ever seen a plunger tip that is broken on the end but hasn't completely fallen off yet? Guess why that tip is broken? That's right, someone forgot to pull the plunger back in order to clear the edge of the playfield. Take a look at the picture below to get an idea of where the release lever is in relationship to the coin door opening.
(click picture to enlarge)