Page numbers are fields.
Page Numbers in Word 2007-2016

Keep in mind that you have at least two or three different headers. You will need to change each, perhaps in each section.

Work in a COPY.

Turn on display of non-printing characters and you will see that you have two separate paragraphs, one for your page number and one for your text. Delete the page numbers currently there and the containing paragraph. Move to the end of your running header text and press the Tab key, then press Ctrl+Shift+P (Cmd+Shift+P?) to insert a page number field.