In Designer, you need to un-attach that page  from the master  page, then attach
it to a different one. (highlight the page in folder view, go to the format
menu, select masterpage)

It gets complicated - do a few test pages first!

Another option  is to edit  the code - for advanced users only.

You are trying to edit  a publishing page. The masterpage setting for these
pages is done through inheritance on the layout page  that the publishing page
uses for display. Since layout  pages get their master  page setting by
inheriting from a LayoutPage object, where the master page is hard coded to use
the Site Master, you can't change  the master page used by the publishing page or
layout page in sharepoint  designer.

Normal, non-publishing pages (the ones not stored in the pages document library)
can be edited  to have a custom master page setting.