Assuming there are no trick questions hidden here, Dan has it right, that the source impedance is irrelevant. The only goal is that v_out be one half of v_in, so, by the voltage divider rule the simplest change to realize this is to modify R2 to be the same resistance as R1.
As an aside, the symbol used for the voltage source appears to be that of an ideal voltage source and an ideal voltage source, by definition, has zero output impedance. But, as Dan teaches, this is irrelevant to the task. You could use any voltage source with any output impedance and as long as R1=R2, then v_out will be one half of v_in.
As for how else it could be done, you could modify the value of both resistors to any other resistance, as long as they remain equal.