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

int main()
{
    FILE *fp;
    char ch;
    fp=fopen("D:\\filename.txt","r+");
    if(fp==NULL)
    {
        printf("Can't open the file!\n");
        exit(0);
    }
    while((ch=fgetc(fp))!=EOF)
    {
        if(ch>='a'&&ch<='z')
        {
            ch=ch-('a'-'A');
        }
        else if(ch>='A'&&ch<='Z')
        {
            ch=ch+('a'-'A');
        }
        fputc(ch,fp);
    }
    fclose(fp);

    return 0;
}

#include <stdio.h>

int main(void) {
    FILE *fp, *fpTmp;
    char ch, tname[80];
    fp = fopen("filename.txt", "r");
    tmpnam(tname);
    fpTmp = fopen(tname, "w");

    if(fp == NULL) {
        printf("Can't open the file!\n");
        return 1;
    }

    while((ch = fgetc(fp)) != EOF) {

        if(ch >= 'a' && ch <= 'z') {
            ch -= 32;

        } else if(ch >= 'A' && ch <= 'Z') {
            ch += 32;
        }

        fputc(ch, fpTmp);
    }

    fclose(fp);
    remove("filename.txt");
    fclose(fpTmp);
    rename(tname, "filename.txt");
    return 0;
}