ASUS UK is releasing Android Wear 2.0 for the ZenWatch 3 according to its official Twitter page, which responded to a question from another user recently stating that the update actually started rolling out to ASUS' third-generation Android Wear smartwatch as of yesterday. This means there are potentially a fair number of users who already have the software update installed on their watches, but like with any update whether it's for software of a device or for an application or game, the update may take some time to hit all users in the region, so those who haven't seen the update alert for their own ZenWatch 3 yet will just have to sit tight a little bit longer, or they could also check manually to see if the new software is available for download by going to the settings menu on the watch and scrolling to find the software update option.
All that said, it's important to remember that this is likely only for the ZenWatch 3 in the UK as there hasn't been any mention of the update coming to the watch for other regions, not so far at least. Since this is Android Wear 2.0 software there are going to be some noticeable change to the user experience and to the visuals a little bit, but not so much that it will be unrecognizable, and all of the changes are essentially the same as any other watch since Google doesn't let OEMs modify the software with their own UI.
As for what those changes are, the obvious new change will be the ability to download and install apps directly on the watch as opposed to installing an app on the connected smartphone and then waiting for the watch to install a companion version of that same app. Now apps will be full-fledged smartwatch versions. In addition to the Play Store, notifications are going to be handled a little differently as well, as you'll swipe left or right to get rid of notifications instead cycling through them, while swiping up and down cycles through and tapping on them lets you see more information about a specific notification card. Sadly, the update doesn't come with Android Pay support as the watch doesn't have the necessary NFC.