If you are trying to change the value of the pointer (ie the memory location the pointer is pointing to) from within the function you need to pass a 'pointer to the pointer' to the function. Dereferencing this one level will allow you to change the memory location pointed to:

void main(void)
{
char *ptr;

Fn(&ptr);
}

void Fn(char **p)
{
*p++; //Increments pointer in main
}