Error in conversion of upper and lower cases

Below code is to convert upper case to lower case and vice vers?

  if(s1.charAt(i)>=97 && s1.charAt(i)<=122){
     s1.charAt(i)=s1.charAt(i)-32;
  }
  else if(s1.charAt(i)>=65 && s1.charAt(i)<=90){
      s1.charAt(i)=s1.charAt(i)+32;
  }

Please refer above and help what is the issue with this program?