DC remains constant with time. I f yes, then how could the magnitude of voltage and current might change with time since dc current flows in one direction?

In the AC generator, there is no DC source used. The coil is rotated manually or by some external force like, wind, flowing water, etc.

 

This rotating coil is acted upon by a magnetic field which induces an alternating emf. Hence, in this way the voltage and current change with time.