Should I try to clean a ww2 german pocket watch or will it destroy the value? Also how should I clean it?

It's stamp .800 silver

I would leave it alone. The tarnish is part of its character.
Paul

Unless it is in really poor condition on the outside I too would leave the patina on the case and dial and hands as they are a part of its history. I would have no compunctions, though, about cleaning the movement inside and would recommend it even as trying to run the watch with old oil inside will cause it to wear very quickly since that old gummy oil is full of dirt and dust that will act as an abrasive agent. And going to an unknowledgeable jeweler or worse one of those mall kiosk places that do "cleaning" for $35 is as bad as not cleaning it at all. All those places do is dunk the movement into a sonic bath and spray some oil on it. What you need to find is an qualified and authentic watchmaker who is able to take the movement apart into each of its component pieces, then clean each one fully, then inspect them and replace any worn parts, then put it all back together and regulate its time. That will cost some money since you would be paying a highly skilled craftsperson for many hours of their time, expertise, specialized tools, and to find or make parts.