First, I would ask why -- 400A seems a hell of a large service capacity.
But here is what is basically required:
1. replace the wires coming from the electric pole into your house with ones that can carry 400A. Probably #000 size. The transformer on the pole might need to be upgraded too, but the electric company will do that.
2. You may need to replace the electric meter & meter housing, too.
3. replace the main panel inside your house with a 400A panel. Try to get the same brand, so your existing breakers can be used in the new panel. This is likely to be a physically larger panel, so much of the work might be just fitting it into the location where the current panel is. And then shutting off all the electrical circuits, and moving them all to the new panel.