There are retainer keepers that lock around the valve stem and sit inside the I.D. of the retainer. Are the heads are the motor or off? If you are chaging seal I will guess they are on the motor still. You will need to make sure both valves are closed and hook up an air line to that cylinder to keep the valves from falling in the cylinder. You will need a special tool to remove the retainers. It will mount to the the rocker bolt/stud location. It will help if you give the retainer a few taps with a soft hammer to help free them up. Then just remove the spring and replace the seal and repeat the process backwards. This time tap on the retainer to make sure the retainer seats. If the heads are off the car you need a big c-clamp tool for removing the valve springs.
If the head is on the car, you will need to:
1.) Turn the engine over so that both valves on the cylinder you are working on are closed.
2.) Pressurize the cylinder with compressed air to keep the valve from dropping into the cylinder.
3.) Place a large socket (1" dia) on top of the spring and give it a firm wrap with a hammer. This will loosen up the locks.
4.) Use and over head valve spring compressor to compress the spring so that you can get the locks out from around the valve stem.
5.) Lift off the valve spring and retainer.
6.) Remove valve seal.
7.) While holding end of the valve stem so it does not drop, remove pressure from cylinder.
8.) Check valve stem for side to side slop in guide. If sloppy, remove heads and have new guides installed.
9.) Repressurize cylinder, install new valve seal and reassemble.
10.) Adjust valve
Don't go crazy with the hammer rap like I did. I dislodged a lock and had it put a burr on the valve stem. I didn't notice and ended up dragging the burr through my nice new valve guides cutting a small groove. The groove was only one or two thousands...but still.