On iOS, holding down the Like/Thumbs Up icon will produce the new selections, or extensions, and sliding your finger over to the icon of your choice will select it. On the desktop, simply hovering over the Like button will give you the variety of selections from which to click to choose.
Updates will show how many of each reaction have been selected by readers. And, for now, no matter which reaction you choose, it will imply to Facebook you want to see more of that type of content in your newsfeed.
Should be interesting!
For further reading: