you can do it like this
string LastYearString = dropdownlist.Text.ToString().Substring(2,2);
int CurrentYearpPart = Convert.ToInt32(LastYearString);
int NextYearPart = CurrentYearpPart + 1;
string FiscalYearToDisplay = "Apr'" + CurrentYearpPart + " May'" + CurrentYearpPart + "June'" + CurrentYearpPart + ",July'" + CurrentYearpPart + " Aug'" + CurrentYearpPart + " Sep'" + CurrentYearpPart + " Oct'" + CurrentYearpPart + " Nov'" + CurrentYearpPart + " Dec'" + CurrentYearpPart + " Jan'" + NextYearPart + " Feb'" + NextYearPart + " Mar'" + NextYearPart + "";
label1.Text = FiscalYearToDisplay.ToString();