reverse after evert two digits

Revewrse will always just rverese the complete arry - string, int, it doesn't matter:

1 2 3 4 5

becomes

5 4 3 2 1

If you want a "special swap then you have to implement it yourself - which is not so simple...

string input = "AbCdEfGh";
StringBuilder sb = new StringBuilder(input.Length);
for (int i = 0; i < input.Length / 2; i++)
    {
    int index = i * 2;
    sb.Append(input[index + 1]);
    sb.Append(input[index]);
    }
if (input.Length % 2 == 1)
    {
    sb.Append(input[input.Length - 1]);
    }
string output = sb.ToString();

The solution 2 is very good.
An alternate method is

string s = "AbCdEfG";
char[] chars = s.ToCharArray();
//Create keys array from the indices of characters 0 1 2 3 4 5 6 =>  2 1 4 3 6 5 8
int[] keys = Array.ConvertAll(chars, x => {int ind = s.IndexOf(x); return ind % 2 == 0 ? ind+2 : ind;});
//Use the Sort method of Array class to sort chars array using keys array
Array.Sort(keys,chars);
string alternateReversedString = new string(chars);

//Or 

string s = "AbCdEfG";
char[] chars = s.ToCharArray();
//swap adjacent characters
char temp;
for(int i = 1; i < chars.Length; i += 2){
   temp=chars[i];
   chars[i]=chars[i-1];
   chars[i-1]=temp;
}
string alternateReversedString = new string(chars);

//The alternateReversedString will be
//bAdCfEG