Hey armchair112,
If I understand this correctly, you want to download the Messenger app on your Mac but you're required to install macOS 10.12 or later and you want to know how you can install this macOS version. Is that right? If yes, then I'm sure I can help you.
In order to install macOS 10.12 or later, you'll need to check if you meet the minimum system requirements. Here's an article that lists the system requirements to install macOS 10.12 (Sierra): How to upgrade to macOS Sierra
If your Mac meets the system requirements, you can search for macOS Sierra through the App Store app and download it to install the software.
I hope this helps answer your question and let me know if you have any additional questions for me.
Cheers!