Choose Format - Arrange - Send to Back (LibreOffice Writer, LibreOffice Calc).
Choose Shape - Arrange - Send to Back (LibreOffice Draw).
Shift+བརྡ་བཀོད་ ཚད་འཛིན་+ཕབ་རྟགས་ཀྱི་ཨང་རྟགས་ (LibreOffice Impress, LibreOffice Draw)
Open context menu - choose Arrange - Send to Back (LibreOffice Impress).