C strings of letters shift

Function is: all the letters in the string s rewritten to the next letter of the alphabet, the letter z rewritten letter a. Still requires capital letters uppercase letters, lowercase letters remain lowercase letters, other characters do change.

Please write function chg (char * s) required to achieve the program, after calling the function readwriteDAT () the output to a file bc1.out in.

For example: s the string is any original content: Mn.123Zxy, this function is called, the result is: No.123Ayz.
#include <conio.h>

#include <string.h>

#include <stdio.h>

#include <ctype.h>

#define N 81 void readwriteDAT();

void chg(char *s)

{int I;

for(I=0;I<strlen(s);I++)

if(s[i]‘z’||s[i]‘Z’)s[i]-=25;

else if(s[i]>=‘a’&&s[i]<=‘y’||s[i] >=‘A’&&s[i]<=‘Y’) s[i]+=1;

}

main( )

{

char a[N];

clrscr();

printf("Enter a string : "); gets(a);

printf("The original string is : "); puts(a);

chg(a);

printf("The string after modified : ");

puts (a);

readwriteDAT() ;

}

void readwriteDAT()

{

int i ;

char a[N] ;

FILE *rf, *wf ;

rf = fopen(“bc1.in”, “r”) ;

wf = fopen(“bc1.out”, “w”) ;

for(i = 0 ; i < 50 ; i++)

{

fscanf(rf, “%s”, a) ;

chg(a) ;

fprintf(wf, “%s\n”, a) ;

}

fclose(rf) ;

fclose(wf) ;

}