You can edit the layout of master pages at any
time; changes you make are automatically reflected on all pages
with that master applied.
Note: When you override or detach
a master page object on a particular page, that object may not update
to reflect changes made on the master page.
InDesign automatically updates any pages using that master.

Use multiple views to see the results of master
edits. Choose Window > Arrange > New Window,
and then choose Window > Arrange > Tile
Horizontally or Tile Vertically. Set one view to a page and the
other view to the master applied to that page. Then edit the master
and watch the page update.