Change value of upvalue from within function

[Eric Tetz <erictetz@...>]

If I have a C function called by Lua. It has an upvalue and I'd like
to change it's value. Possible?

   static int closure_that_has_upvalues( lua_State* L )
   {
       ...
       // how can I change the value of, say, the upvalue at
lua_upvalueindex(1)?