(My previous answer was probably wrong, so I deleted it and am giving this one, which is better.)
Andrew Swann is absolutely right that we need a minimal working example; but let's see one way you could get this done. Assuming you're using the
article class, and your page number is centered in the footer:
This will do what you want, but it will leave page numbers elsewhere alone. It will also work through commands that change the page number's format, if you're using a document class which has such commands, such as
fancyhdr documentation is quite good, and will give you a great deal more information about how else you can format running headers and footers.