#include <stdio.h>
#define RECORDLEN 6
int main()
{
    char buf[]="6789";
    FILE *fp;
        fp = fopen("test.txt","r+");
        fseek(fp,RECORDLEN,SEEK_SET);
        fprintf(fp,buf);
        fclose(fp);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>

#define RECORDLEN 6

int main(void)
{
    long pos;
    char buf[] = "6789", *p;
    FILE *fp = fopen("test.txt", "rb+");
    if (fp == NULL) { puts("can't open test.txt"); return 1; }
    fseek(fp, 0, SEEK_END);
    pos = ftell(fp);
    p = malloc(pos - RECORDLEN);
    if (p == NULL) { puts("out of memory"); return 1; }
    fseek(fp, RECORDLEN, SEEK_SET);
    fread(p, 1, pos - RECORDLEN, fp);
    fseek(fp, RECORDLEN, SEEK_SET);
    fputs(buf, fp);
    fwrite(p, 1, pos - RECORDLEN, fp);
    free(p);
    fclose(fp);
    return 0;
}