High-order harmonic generation (HHG) from aligned CO₂ is studied within the framework of the strong-field approximation (SFA). Our results are in qualitative agreements with recent pump-probe experiments. The experimentally observed inverted modulation in HHG signals as a function of pump-probe delay time has previously been attributed to the quantum interference from the two oxygen centres. Our results, however, indicate that this is not necessary for the inverted modulation. The angular dependence of the HHG and the evolution of the HHG yield as functions of delay time are influenced strongly by the depletion of the ground state and, therefore, are sensitive to the probe laser intensity.