I've never done a Buick 6 before, so this may not apply, but here's how to do it on an SBC with pedestal mount rockers.
You need to hold the valves up in the cylinders. There's a couple ways to do it. Some guys pull the plugs and thread some cotton rope in through the spark plug hole, then slowly turn the engine over by hand until it stops. The rope on top of the piston should hold the valves up when you remove the springs. You can do the same thing with vinyl or butyl tubing. I use an adapter for my air hose that screws into the spark plug fitting. Normally, I can set the regulator on my compressor to 30 PSI or so and that is plenty to keep the valves in place.
Once you've got a way to keep the valves up, you'll need a small valve spring compressor. Advance sells one for $20 that looks like a 2 jaw puller that's pretty compact. I was able to get to all the springs on my Corvette with it, and the engine bay in there is quite a bit tighter than the Buick. You'll put the compressor on the spring and tighten it down. Normally, it will pull the bottom of the spring up, and you'll have to smack the top of the spring compressor slightly with your palm to get the spring to drop off the keepers. Before you smack it though, put a shop magnet, or one of the antenna style expandable magnets on top the spring. Usually, when you smack the compressor the keepers will stay on the valve stem, but not always. Once the keepers are exposed, move them around with the magnet until they lift free. Then loosen the compressor and pull the spring off. Be careful, some valve springs, when compressed contain a BUNCH of energy. Enough to kill a person sometimes. Try not to let the valve spring slip out of the compressor, it's got the potential to shoot out and hurt someone.
Now you're ready to set up the new spring. Install the shims on the head if the manufacturer calls for them. Loosen the compressor all the way, and install the new spring into the compressor, then tighten the compressor down. Be sure to install the hat, top plate, shims, etc... on the spring. Slip the spring and compressor over the valve stem as a unit. Install the keepers and loosen the spring compressor. If you are using different retainers, be sure to use the proper keepers.
Like I said, that's all based on pedestal mount SBC rockers. The Buick stuff could be completely different seeing as it's a shaft mount system. I don't 'think' that swapping the springs would be much different, but I've painfully discovered lately that the Buick 6 is considerably different than the SBC.
I'm sure this has come up before, some searching may yield a write up somewhere.