#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;
}