There’s an Android Wear system software maintenance release available for Polar M600 users.
This update fixes the problem of the optical heart rate measurement not working after M600 users updated their Android Wear system software from version 1.5 to 2.0 (build number NWD1.170806.001). But we recommend that all M600 users install this maintenance release even if there wouldn't have been any issues with the optical heart rate measurement on your M600 as this release includes the latest security updates for the Android Wear system software.
The system software update will roll out gradually within 2 days to all Polar M600 smartwatches. You’ll be notified directly on your M600 when you’ll be able to upgrade your smartwatch. You can download the update to your M600 by plugging the smartwatch in to charge while the M600 is connected to a Wi-Fi network or to your phone when your phone has an Internet connection. See the support documents How do I update my M600 system software to Android Wear 2.0? and How do I update my M600 software? (Android Wear 2.0 and up) for further instructions.
After successfully updating the software the build number on your M600 will be NWD1.170904.001. See the FAQ How can I check my M600 software versions? if you need instructions on how to check which version of the build you have on your M600.