C programming to convert upper case string into lower case string and vice versa without using builtin function?

#include<stdio.h>

#include<conio.h>

void main()

{

char ch;

clrscr();

printf("Enter a Character");

scanf("%c",&ch);

if(ch>=65 && ch<=90)

ch=ch+32;

printf("Upper Case =%c",ch);

getch();

}