Once you purchase the POD 2 you can now go to the Device Page in the COROS app and add as a device. Once the POD 2 has been added, your watch will alert you that a new firmware is available for your APEX 42/46mm. You can then download that updated firmware and use the POD 2.
You can also use this link to enroll in public beta firmware to initiate that download: https://api.coros.com/coros/join-beta/1/7dmkx0EOjk/en