How can I replace the first occurence of a word in a file?

void replace_first(char* oldWord,char* newWord)
{
    printf("f\n");
    FILE * fPtr;
    FILE * fTemp;
    FILE* f;
    char path[1000];
    char file[BUFFER_SIZE];
    char buff[BUFFER_SIZE];
    char buff_2[BUFFER_SIZE];
    printf("%d\n",num_files);
 //   char oldWord[100], newWord[100];
    f= fopen("files_to_be_scanned.txt", "r");
    while (!feof(f))
    {
    	fgets(file,sizeof(file),f);
       // file[strlen(file) - 1] = '\0';
    	printf("\nfile is %s\n",file);
        strcpy(path,file);
        printf("path is %s\n",path);
        path[strlen(path) - 1] = '\0';
        fPtr  = fopen(path, "r");
        if(fPtr == NULL)
        {
            perror("error in main files");
        }
        fTemp = fopen("replace_4.txt", "w");
        if(fTemp == NULL)
        {
            perror("error in replace file");
        }


        /* fopen() return NULL if unable to open file in given mode. */
        if (fPtr == NULL || fTemp == NULL)
        {
            /* Unable to open file hence exit */
            printf("\nUnable to open file.\n");
            printf("Please check whether file exists and you have read/write privilege.\n");
            exit(EXIT_SUCCESS);
        }



     while ((fgets(buff, BUFFER_SIZE, fPtr)) != NULL)
     {

        printf("beforeeeee %s\n",buff);
        char *pos,temp[BUFFER_SIZE];
        int index = 0;
        int owlen;
        owlen = strlen(oldWord);
        while ((pos = strstr(buff, oldWord)) != NULL)
        {
            first_flag=1;
            // Bakup current line
            strcpy(temp, buff);

            // Index of current found word
            index = pos - buff;

            // Terminate str after word found index
            buff[index] = '\0';

            // Concatenate str with new word
            strcat(buff, newWord);

            // Concatenate str with remaining words after
            // oldword found index.
            strcat(buff, temp + index + owlen);
            printf("TTTTTTTTTTTT %s\n",buff);
            fputs(buff, fTemp);
        }

        if(first_flag==1)
        {

            first_flag=0;
            break;
        }
       // printf("Afteeeeeeeeeeer %s\n",buff);

     }
       /* Close all files to release resource */
        fclose(fPtr);
        fclose(fTemp);


        /* Delete original source file */
       int del = remove(path);
       if (!del)
       {
            printf("The file is Deleted successfully\n");
       }
       else
       {
            printf("the file is not Deleted\n");
            perror("error in deleting file ");
       }


        /* Rename temp file as original file */
        rename("replace.txt", path);

        printf("\nSuccessfully replaced first occurrence of '%s' with '%s'.\n", oldWord, newWord);

    }

}

main replace -f  I hoda  -- file1.txt file2.txt

if(first_flag==1)
        {

            first_flag=0;
            break;
        }