Disassembly of the cooler is rather straight forward. Start by removing the bolts that attach the front housing to the base of the cooler. The front housing bolts are not the ones on top, except for one bolt, but rather the ones the level below. I'v attached a picture of the (pic) front of the EGR cooler with highlights showing some bolts to remove but there are more that you can't see in this angle. Once these bolts are removed, using just a little force, you can separate the housing from the base.
Next remove the rear crossover cover by removing the bolts. Remember the location of the studs. This cover should be easily removable. The final step of disassembly is forcing the EGR cooler core out of the base. You can accomplish this by finding two large diameter rods that fit down into the core from the side that had the cover on it. This rod will push on the fins inside of the core to remove it. The rods can be metal (exhaust pipe works), brass, or wood (old hammer handles).
Stand on the base and alternate between hitting each rod until the core becomes loose enough to remove it completely. Keep track of which (pic) metal EGR cooler gasket goes on the inside of the core to the housing.
Be sure to clean the base housing, the crossover cover, and front housing. Carbon builds up in all three of these locations. Usually a small brush and a water house work pretty well. The parts do not have to be spotlessly clean. Also, do not forget to remove and replace the (pic) EGR Cooler seals on the back side.
Once everything is all cleaned up and ready to go, lube the seals you installed and install the inner metal gasket first on to the base before inserting the EGR cooler core. Push the core down into the base but once you hit those seals you will need to find something flat and soft (like a wooden block) to finish pushing it into the base. Do not hammer it into place, it will likely bend the cooler. It should slide in easily when you bear your weight on it. Remember to line up the roll pins with the holes. Once the core is in place, remember to place the second gasket on before reattaching the front housing assembly. You get new seals that fit into the valve cover so be sure to install those as well. Now reinstall your EGR cooler.
Follow the refill procedures listed on my other article Engine Coolant Refill Procedure for both the primary and secondary cooling system, however the 6.7L Powerstroke does a pretty good job of bleeding the air from it's cooling system naturally.
Clear the codes, test drive to make sure P0401 does not return. You are set to go.