On most diesels the injector pump can be shifted to change the timing. Just loosen the mounting bolts, shift the injector with a crowbar, and then re-tighten the bolts.

On some diesels it is necessary to disconnet the injector fittings. This is because the pump might shift a bit, but will quickly spring back.

On my Toyota 2.4L I can shift the timing by loosening the two front mounting bolts and the two rear mounting bolts that attach to a steel plate. If I pull the pump away from the engine the pump gets advanced. Inward is retard.

On my brother's Mercedes the opposite is the case. He has to pull the pump away from the engine to retard the timing.

Whereas I can change the timing without removing the fittings, he has to take them off.

Before you change the timing, mark the existing timing spot. That way you can always go back to the factory setting. Having said that, a timing mark on a flange is only a starting point. Even in manufacturing there are various clearances that vary between vehicles. Further, after the timing belt has been used for some time it stretches. This will then to retard not only the camshaft, but also the injector timing.

Try shifting the timing a fair bit the first time. Then if it proves to be too much, you split the difference from where it was originally. If that is still too much, then split it again. You want to find out what is too much of a shift. If you start with small increments it may take you a while to get to the too much stage, and then you still have to go back again. Just move it a fair amount and then start splitting the difference.