Automatic increment a letter in headers

At the end of my document I have some Annexes labeled ‘18. Annex A’, 19. ‘Annex B’ , … styled as Heading1.
How can I autoincrement the letter A,B, at the end of heading independent from the chapter number, so that, should I shuffle the annexes around, the annex letter is changed to maintain the alphabetical order.

Would you prefer to have “Annex A - Title of annex”, “Annex B - second annex”? If so, tell through editing your question, not with an answer. Otherwise, tell why you have two “numberings” in your annexes (one for the chapter numbering, one for the annex lettering).

Note this is a common question because Heading 1 can have only a single way of numbering the chapters (you can’t switch from number to letter).

I support @ajlittoz in suggesting you to turn your double numbering scheme into a normal numbering; but in case you really need such a strange scheme, you could use a dedicated Number Range field, where you create a field with name, say, AnnexNumber, and value set to AnnexNumber+1; set format to required numbering style; and insert copies of the field into your annexes. The fields will automatically renumber as you move the paragraphs back and forth.