The only method that I'm aware of that does not involve removing the head follows: 1. Hook a regulated air hose to a fitting that is sealed to the cylinder. On a diesel, you'll either have to use the injector location, or a glow plug location. Either way you have to put a constant air pressure into the cylinder with both valves closed. A compression tester hose works great for this. You will need an air compressor to supply the air.
2. Once you have the cylinder under pressure, (you will hear air hissing into the crankcase, but don't worry about it.) You can use a small jaw type valve spring compressor to get the spring collapsed enough to get the valve retainers out and remove the spring.
3. Once the spring is out of the way, replace the seal. Be careful that you don't push the valve down and release the air pressure.