You'd create a list containing 01, 02 etc and another list containing A, B etc.
When translating forwards ie from 01 to A:
outputBox.Text = list2[list1.IndexOf(inputBox.Text)]
When translating backwards ie. from D to 04:
outputBox.Text = list1[list2.IndexOf(inputBox.Text)]
Explanation:
list1 contains 01, 02, 03, 04, 05 etc
list2 contains A, B, C, D, E etc
When converting from eg. 01 to A:
• Get its index in list1
• Set outputBox.Text to the item in list2 at the same index.