If your thesis has appendices, place this command before the first or only appendix:
\appendix
After that point, each \chapter will
be numbered with an uppercase letter, e.g., Appendix A,
Appendix B, and so on. Sections will display section
numbers that start with the appendix letter: for
example, sections of Appendix A will be numbered
A.1, A.2, ….
For an appendix that appears as part of a separately
published chapter, see Section 8.3, “The chapterappendices environment”.