I'm signed up for the Public Beta and was able to get the Beta 2 following the directions others mentioned.
Downloading now. As a Public Beta tester, my pet bug, which I've reported repeatedly to Apple, had been the complete failure of Swiftkey to come up whenever I do anything that equates to a share. Whether I was sharing a link or a photo or, just trying to jump across apps to a message or email, I got no keyboard at all. If I made an email draft, I could open the draft and keep working with Swiftkey, but there was a major bug with third party keyboards; in messages, there was nothing I could do, aside from Copy what I was attempting to share, and pasting it into the message.
Now that Beta 2 is live on my 5S, I am thrilled to discover that this bug appears squashed!
The next apparent third party keyboard bug I'd like to see fixed is getting kicked out of the 3rd party keyboard randomly, reverting back to the iOS keyboard. Time will tell if this update addresses that, which isn't as critical as zero keyboard functionality.