How are you trying to edit the Header or Footer?
Is this a case of the White Space between pages being hidden?
If you move the mouse cursor to the top of the page until it turns into two vertically opposed arrows, does tip "Double click to show white space" appear?
I edit the header/footer by double-clicking in the visible zone. The footer currently contains an incorrect reference.
I'm familiar with the White Space issue and it's not hidden.
I can see the existing footer text until I double-click the footer zone. The same applies to other Footers using the Next and Previous section buttons.
A suggestion, turn on display of non-printing formatting marks. Any section breaks?
Showing non-printing formatting marks in Microsoft Word
The footer that prints and is displayed may not be in the section you are editing. When you click in the footer area this should not happen, but try it anyway.
Each section in Word has three headers and three footers, whether you see them or not. Each page in Word may have multiple sections. Documents converted from Wordperfect or the web often have multiple sections in a single page.
Sections / Headers and Footers in Microsoft Word 2007-2013