I would be very hesitant to do this myself. I bought a Summaron, with the round prism out of alignment--it seemed to focus correctly, but the vertical alignment was off, as in your lens.
I took it to my local camera shop proprietor, an old fella in South London who's been selling Leica gear for decades. He bundled the lens off to his regular repair guy, who is quite competent and does Leica work regularly. Unfortunately, this is *not* a usual job, and it turns out it may be one of the most difficult things regarding Leica lenses to fix properly.
Three times the lens went off for adjustment. All three times it came back in perfect alignment, *but* within minutes after I had put the lens on the camera, and before I even left the shop, the prism had gone clearly out of alignment again. And I wasn't banging it on the counter or anything. It was the damndest and most frustrating thing.
After the third failure of the repair to take permanently, I finally sent it off to the dean of UK Leica repairmen, Malcolm Taylor. He finally got it licked.
Malcolm kept the lens for nearly two weeks. He explained that the screws that Leica originally used in the lens were often not stout enough to keep the prism permanently in alignment. He re-tapped the screw holes and used longer and slightly stouter screws. He lacquered them into place, and then allowed the lens to rest for a few days while the lacquer dried thoroughly, making sure it was exposed to a variety of temperatures, to ensure the prism wasn't going to shift out of position as the metal bits expanded and contracted.
Once all was to his satisfaction, he sent the lens back to me. Malcolm did caution me that even with his efforts, the prism could conceivably shift again, especially if it took a bit of a knock. But ever since, more than two years later, the lens has been fine--knock on wood.
I'd take Youxin's advice and send it off to DAG or someone else who has experience in this particular ajustment.
