In page view, the internal margins are displayed in green. In the upper-left and bottom-right corners you will find a small green square. You can move this square to adjust the margins and the staves will follow accordingly.

You will also find "+" and "-" buttons to the right of each staff system. They can be used to add/remove one measure in that system, the following measures being shifted accordingly into the following systems. Notice that these buttons will not add new measures to the staff, they only shift the existing measures.

Similarly, the "+" and "-" buttons displayed at the bottom left of the page, are used to add/remove a system on that page. The following systems are shifted accordingly into/from the other pages.