Choose Format - Arrange - Send to Back (LibreOffice Writer, LibreOffice Calc).
Choose Shape - Arrange - Send to Back (LibreOffice Draw).
Shift+Ajaja Ctrl+mallattoo hir'isu (Imprassii LibreOffice, Fakkassaa LibreOffice)
Open context menu - choose Arrange - Send to Back (LibreOffice Impress).