Using the Shape Editor Tool , drag a box around the bottom point to marquee select it. On the Infobar at the top of the screen, press the Break at Points icon (shown to the right of the Shape Editor Tool icon).

TIP: Not sure what something is called? Let the cursor rest over the button or icon for a few moments and a Tool Tip will appear displaying the name.

Break at Points opens the closed path and creates a start and end point of the open path.

Select the path with the Selector Tool (the arrow icon), then using the Shape Editor Tool, separate the points as shown.

TIP: Because the start and end point are one atop the other, it is difficult to select just one. Press the Tab key to tab to one of the end points, then use the arrow keys on your keyboard to nudge the point in the correct direction.