On the meter bridge, look at the back. There are aceews around the perimeter of the back panel of the meter bridge. Remove the screws and the panel comes off. Then you’ll be able to see inside the bridge. The meters are held to the face of the bridge in pairs with a bracket fastened with screws. Remove the screws and the meters can be pulled out through the open back of the bridge. There are wires soldered between each pair of meters so the whole bundle comes out together u less you want to desolder the wires. Once the meters are out you have to get the lenses off the meters to access the lamps. The lenses are held on with clear tape. Take a razor and slice the tape along the joint where the lense mates with the meter body. Just do it in three sides. You can leave the tape intact on one side and that becomes something of a hinge for the lenses…flip it open. If the lamps are original they’ll be pop-out type. Then you solder the new ones in. Make sure to orient the lamps the same way in each pair. You’ll notice the leaded lamps have a smaller end at one end. Orient them the same way. This is all from memory. It’s been 10 years or so since I’ve had an M-500 meter bridge opened up, and I’m fuzzy on some details. But hopefully that gives you enough to go on. You know, the service manual has exploded vies of all this for reference. That might help too.