Take the backglass off, and put it in a safe place...
Reach down in the neck area; I believe you have two sliding metal pieces that slide over the top of the plexiglass piece that covers the score displays; you'll need to pull these back (towards the back of the game).
Once those are pulled back, you can then lift up the plexi that covers the scoring displays. I had a Centaur II for a while, so had to do this a few times.