int update_pic(User *user, const char *filename) {

    if (strlen(filename) > MAX_NAME) {
        return 2;
    }
    FILE *output_file = fopen(filename, "w");
    if (output_file == NULL) {
        return 1;
    } else {
        strcpy(user->profile_pic, filename);
        return 0;
    }
}
Change the filename for the profile pic of the given user.