If you're among the crowd that both installs Android developer previews and also owns an Android Wear watch, you've probably noticed some of the apps that belong on your watch have been missing. This happens because the stock apps included with the developer previews are missing the micro-apks for Wear. As the official release of Android 7.0 draws near, Google has been slowly releasing app updates that can install over the stock versions on the developer preview. And as of yesterday's stream of updates, Google Messenger joins that list.
So far this version doesn't appear to make any other significant improvements over previous updates, but keep an eye out for things that may not be obvious. You can grab the latest update from APK Mirror at the link below, or wait for it to roll out to phones over the next few days.
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.