At the top left corner of the drawing pane, where the rulers meet, there is a button showing crossing lines. Click and drag from that button. The reference lines (horizontal and vertical) for the new origin should follow the mouse pointer while you drag, and when you "drop" (release the mouse button), the rulers should reflect the new zero point.
Note that positive Y direction is still downwards. Everything above zero will have a negative Y coordinate. I don't know any way to change that.