C++14, 30 bytes

As unnamed lambda (allowed in Python):

[](decltype('B')*c){*c='C'-2;} //'c'-2 for lowercase

//Usage:
#include <iostream>
main(){
  char c;
  [](decltype('B')*c){*c='C'-2;}(&c);
  std::cout << c << std::endl;
}

33 bytes

Function that assigns to its parameter:

int f(decltype('B')*c){*c='C'-2;}

//Usage:

#include <iostream>
main(){
 char c;
 f(&c);
 std::cout << c << std::endl;
}

Pretty hard to do it otherwise since no #include, return or auto because of U. Modifying parameters for return values seems ok Meta