#include <stdio.h>

void foo(char* bar) {
    bar[0] = 'G';
}

int main(int argc, char** argv) {
    char str[] = "FOO";
    foo(str);
    printf("WORD: %s\n", str);
    return 0;
}
$ gcc test.c -Wall -o test && ./test
WORD: GOO