I assume that you are not the creator of this document. If you were, I expect you would know the answer to your questions. This is a somewhat complex document and requires knowledge and skills not required of most Word users.
Headers and Footers are a part of Sections and are contained in the section break that ends a particular section. They can be linked from section to section or not. You can also copy text from one section's headers and footers to those of another section.
Header/Footer Settings Recap (Just read the recap, not the entire page, for now.)
Chapter numbering has prerequisites. Among those are a well-designed numbering system (not just one of the buttons on the ribbon) and section breaks at the beginning of each chapter.
For someone here to assist you in making the changes you need before you read those will take more time for you than will reading the articles linked above. To give context to those, you can take a look at:
Reading the first three links will make it much easier for someone to walk you through any questions you have. The last two are something that might help but are more involved. If you are going to be working with this document much, you probably want to review them, but your immediate questions do not require them.
This post required 4 separate saves to keep from losing material typed while I looked for links. Sorry if that caused any inconvenience.