editing parts

Double click the icon, not single click. This will insert a page break on that measure, forcing it to end the page so a new page starts with the next measure. So this works to make a page break *earlier* than it otherwise would. To get the page break *later* than it otherwise would - to fit more music on a page - you'll need to change your settings make the music smaller, make the spacing tighter, make the margins smaller, etc - because MuseScore is already fitting as much as it can given your current settings.

In this respect, MuseScore is just like a word processor. You can add a page break to end a page *early*, thus fitting *less* on the page than there would be by default. But if you want *more* on the the page, you'd need to reduce font size, line spacing, character spacing, or some other parameter, because by default the program already fits as much as it can given your current settings.

If you want to 'condense' the music to fit one or more additional measures on a page, select the last few systems on that page and then type } several times to reduce the 'stretch' in those measures.

If you need to stretch out the music to move a measure or two onto the next page, either use the page/line break function as Marc has explained, or select a few systems and type { until the music expands to do what you want.