使用CharToArray和foreach来增加或减少用户选择的字母

Near the end of my program I have turned my user input into a chararray called letter. I want to iterate through letter and for each "letter" in the array, I would like to add or subtract the value of shiftCode. shiftCode can be a positive or a negative. I have a small part that functions where it simply adds 1 to the first letter of "letter".Near the end of my program I have turned my use