Try as below code.
string input = "AXBYCZ";
StringBuilder output = new StringBuilder();
char[] characters = input.ToCharArray();
for (int i = 0; i < characters.Length; i++)
{
if (i % 2 == 0)
{
if((i+1) < characters.Length )
{
output.Append(characters[i + 1]);
}
output.Append(characters[i]);
}
}
Input: "AXBYCZ"
Output: "XAYBZC"
Incase length of string is odd number. Then last character will maintain its original position.
Input: "AXBYCZT"
Output: "XAYBZCT"