StringBuilder - Replacing Each Letter Without Reverse Tool

How can I replace each letter for example "abcde" to "edcba" with StringBuilder only and without the reverse tool. This is what I tried:

StringBuilder str=new StringBuilder("abcde");
int indexBegin=0;
int indexEnd=4;
for(int i=0;i<str.length();i++){
str.setCharAt(i, str.charAt(indexEnd));
indexEnd--;
}
System.out.println(str);

The output is:edcde with i understand why its wrong ,the last two letters already swap so it didnt take from the original str.