As well as editing frame content, you can edit master page content further by detaching or linking the master pages from your publication page:
Detaching breaks the connection between the master page and its elements so you can transform frames and alter frame properties (change number of columns, add a frame stroke, etc.), in addition to editing frame content.
Linking lets you edit the master page directly from any publication page so it updates all its associated publication pages simultaneously.
To detach or link a master page:
From the Layers panel, select the master page entry (e.g., 'Master A - 2 Pages') and choose an Edit option from the context toolbar (or right-click the master page's layer entry for equivalent options).
Detached—lets you transform the frame and change its properties independently of the master page's frame. Changes affect only that page/spread.
Linked—changes affect the master page and all other pages that use that frame.
Clicking Finish on the orange information bar to commit your edit. You can also click away from the page to commit.