Choose Format - Arrange - Send Backward (LibreOffice Writer, LibreOffice Calc).
Choose Shape - Arrange - Send Backward (LibreOffice Draw).
Commandបញ្ជា(Ctrl)+សញ្ញាដក(LibreOffice Impress, LibreOffice Draw)
Open context menu - choose Arrange - Send Backward (LibreOffice Impress).