rests

Select whole (measure) rest, press 6, select 2nd half rest, press 5, select 2nd quarter rest, press A-G

But more to the point, that isn't something you would ordinarily do at al. Instead of thinking about "changing" one thing into another, you simply enter the rests along with the notes, left to right, exactly as you would read them. So, go to note input mode, enter the half rest, quarter rest, and quarter note, in that order. To enter a rest, right click or press 0.