Moving a measure

This works basically like a word processor. To end a line early - putting less on it than would normally fit - simply hit Enter to add a break (or use the Breaks & Spacers palette in MuseScore). But if you want to fit more than otherwise would given your current settings, you'll need to figure out what you want to sacrifice to make that happen - either a small font size (in this case, it's the "staff space" size) or tighter spacing. The former setting is in Layout / Page Settings. The latter settings are in Format / Style / Page. You can also override those settings for selected measures using the shortcut "{" (or Format / Stretch / Decrease Layout Stretch).