Moving a measure to previous system

You need to create a space for the notes first? Insert a few blank measures ([Ctrl]Insert or Create -> Bars - Insert Measures) then select, copy then paste.

Maybe you mean there is already music on the previous system, but you want to get more measures there? In which case, you need to make the music smaller somehow, because if there was room for more measures with you current settings, MuseScore would have put them there already. So either use small type over (layout / page settings / space) or reduce the stretch factor (select the measures you want reduced, layout / add less stretch). Or increase page size or margins. Anything that makes it psosible to fit more measures. Once it becomes possible, MuseScfore will put more measures on each system automatically.