string s = string.Format("{0}{1}", s.Substring(1, s.Length - 1), s[s.Length - 1]);
would be better
string s = "stack";
int lengthOfMovingPart = 1;
s = string.Format("{0}{1}", s.Substring(s.Length - lengthOfMovingPart),s.Substring(0, s.Length - lengthOfMovingPart));