Click on the hyperlink you wish to edit.

You'll see a small window pop up next to the link. On the right side of that pop-up is a pencil icon. Click that. 

Then click the gear icon that appears.

In the window that opens, you'll notice a check box labeled 'Open link in new tab.' Check that box, then hit the green Update button.

Make sure to hit the Update button on the page you've just edited to save the change! And voila, the link will now open in a new tab.