In a particular document, go into the footer involved and change it.
Insert > Footer > Edit Footer

Each section has three headers and three footers. If there are multiple sections, each of these in different sections can be linked to previous or independent.
Header/Footer Settings Recap

For future reference, one of the worst ways to insert a page number is to use Insert > Page Number and pick something from the gallery. Among other things, this will overwrite existing headers/footers. Instead go into the header or footer and insert the page number manually. Alt+Shift+P will insert a PAGE field at the insertion point. If you want your page numbers to have different formatting from other text in the Header/Footer, apply a character style to it. That way, if you later want to change it, you can simply modify the style and it will change throughout a document.