You can edit header and footer text on any page on which it appears.
To edit the text:
Place the caret at the place where you want to start editing.
Replace the text or add new text in the usual way.
This process will change the header or footer on all the pages where it appears.
If you want to change the header or footer on some but not all pages, create a new header or footer with the different text and use the Pages dialogue box to apply it to the pages you want.