Insert a section break; format the header or footer differently.

Or use the following field construction

{ IF { PAGE } > 5 { PAGE }  }

You must use Ctrl+F9 for each pair of field delimiters.  If you want the
sixth page to be numbered as page 1, then use

{ IF { PAGE } > 5 { = { PAGE } - 5 }  }

But note that if you need the page numbers to appear in a table of contents,
they will not be correct and in that case you need to use a Section Break as
Jezabel suggested and format the page numbers to start at 1 using the format
page numbers button on the headers and footers toolbar.