In a multi-page view
Click a page either in a multi-page view or double-click it on the Pages navigator.
The active page is shown with a slight border around its pasteboard.
Note: even if you select more pages on the Navigator palette, only one of those can be active page.
In a single-page view
Just click on the page window to make it current.
In the Pages navigator
Double-click a page in the navigator. If it was already in the multi-page view (but out of sight), Fred4 brings it in the view. Otherwise, it opens it and brings in the view.