When you create your documents you tell Scribus how the page should look. You set:
A size
An orientation
The margins
But sometimes, you need some changes in your document. From that, you can consider several options depending on what you need.
Do you want the changes to apply to every new page?
Do you want to modify an existing page?
Do you want to affect the way you'll organize the content of the page?
Do you want to change the page itself?
We could go on with a longer list. But this will help with choosing the best Scribus method.
Mainly, if you want to change the new pages or all the pages of your document, just go to the Document part of the Document Setup (in the File menu). There you can change the setting you want. They are very similar to the New Document dialog, so no extra information is needed.
If you want the new settings to be applied to existing pages, you should think of selecting the All Document Pages and/or All Master Pages checkboxes. Otherwise, the changes...