The removal of the lenses is made easy by lifting the cover on the PJ AFTER you have powered it off (or removing it all together).There are four black nuts located at the corners of the plastic lens housing of each gun; conversely, they could be long, almost bolt-like in appearance. In either case, you need a, um...hang on a sec...
(Insert "Girl from Ipanema" muzak here)
OK, you need an 8mm or 5/16" socket to remove the nuts (and be careful not to lose one into the PJ. This is why I recommend powering down the unit, just in case).
Unfortunately, I don't have the service manual for the BG 800, so I can't help you there. Use the forum search function to look up manual for the Barco. You may be able to come up with the installation and users guide.