There is no update for the 8700's BB Messenger app, other than what comes bundled with the most recent OS. And that's old. Nobody is developing for a legacy device like the 8700 anymore, so what you have is about all there is. It's still a solid device and quite capable with the basic BlackBerry functions, but few of the latest apps will work on it.
At the top of the 8700 Forum
you'll find a link for the most recent OS. Make sure to pick the one specifically for your exact device. Don't use the 8700 version for an 8703, etc.