You can't. If you want something like that, create a class that wraps an primitive and use methods to change it. Or use a static variable but I don't recommend that since you can't control access to it.
"Any fool can write code that a computer can understand. Good programmers write code that humans can understand." --- Martin Fowler
Please correct my English.
There is no such thing as passing by reference, not even for objects. With objects, you pass a copy of the reference. This allows you to modify the object itself, but not the original reference itself.
I agree with Luigi. There's never a good reason to do this. A much more robust way is to call a method that returns a primitive value based on the primitive you passed to it. The caller can then reassign the variable passed with the value returned. Avoid methods with side-effects if you can.
The mind is a strange and wonderful thing. I'm not sure that it will ever be able to figure itself out, everything else, maybe. From the atom to the universe, everything, except itself.