tolower function is not working with me

#include<iostream>
#include<cstring>
#include<fstream>
#include<cctype>

using namespace std ;
int main(){

int counter1 = 0 , counter2 = 0  , i =0 ;
char cstr[500] ;

ifstream fin ;
fin.open("paragraph.txt");
while(!fin.eof() ) {// eof = end of file

    fin.get(cstr , 500 );
    for( ; i<strlen(cstr) ; i++ ) {
        if( cstr[i] >= 'a' && cstr[i] <= 'z' ) {
            counter1++ ;
            toupper(cstr[i]);
        }else if ( cstr[i] >= 'A' && cstr[i] <= 'Z' ) {
            counter2++ ;
            tolower(cstr[i]);
        }
    }
}
cout<<cstr<<endl;
cout<<" capital case : " << counter2 << " <=|=|=> small case : " <<counter1 ;
return 0 ;
}

i have a text in my paragraph file and i want to convert each capital letter with its small case and vice verse. i can't find any error with my codes and the counters work fine so my nothing wrong with my codes