To update the M-2 earphones you need the Earin M-2 app. For Android devices it can be found in Google Play, while for iOS devices it is available in AppStore.
When the earphones are connected to the app, there will be an "Update available" button shown if a more recent firmware exists.
When you start the update, the app will guide you on how to proceed.
Update with Android
Make sure you are paired to both earphones in your Bluetooth list if you want to update both earphones together.
If updating both earphones together, the first will be updated to 99% before second earphone starts updating. Once both have reached 99%, they will do the final step together and go to 100%.
Update with iOs
With the iOs app, you will update one earphone at a time. You start the update for one earphone. When the first earphone is updated, the app will confirm that the first earphone is complete. After that, put back the first earphone into capsule, connect second earphone and start the update for this earphone as well.
Check firmware version
To check what firmware version your earphones have, go to About in the app. Two rows with firmware info will be shown there if both earphones are connected to the app. If update successful, both earphones should now have the same version number, "CSR 2.0.159 NXP 01.08".
Please be aware that both earphones need to have the same NXP version to be able to play together. One earphone will be mute if only one in the pair is updated.