This might do the trick for you
string str = "EXP.120.556"; //120.002.123
str = String.Join(
                    ".", 
                    str.Split('.')
                        .Take(
                                str.Split('.').Length - 1
                             )
                 ) 
                 + "." + 
                 (
                    Convert.ToInt32
                    (
                        str.Split('.').LastOrDefault()
                    ) + 1
                 ).ToString();
So here in the code the first String.Join will join the other part of the string with . except of the last part which you want to increament using Take. Then Convert.ToInt32 will convert the last part of the string to a integer using LastOrDefault and then we add 1 to it and again convert it back to string using ToString