My Mac cannot be updated. It uses a mac OS 10.12 and cannot be updated to macOS 10.13 or higher so the browsers (Goggle chrome, safari, etc) cannot run videos anymore, for instance.
MacBook Air 13″, macOS 10.12
Back up your data, check the application compatibility table, click here, and then on the download link for a higher OS. Any Mac running 10.12 can be upgraded at least as far as 10.13.6; if you encounter a specific issue, describe it in this thread.
Catalina and above won't run 32-bit software and split the functions of iTunes up; if you choose one of them, use Go64 as well.
(250055)
As Niel wrote you can upgrade that Mac to macOS version 10.13.6 "High Sierra" at least, but its Safari version will still be too old for many websites.
You can try Brave or Firefox ESR (Extended Service Release):
https://www.mozilla.org/en-US/firefox/new/
Brave will no longer be updated. Firefox may already on its last release but I am not certain of that. To my knowledge Google Chrome won't work at all.
what to do if my mac is considered vintage and cannot be updated?
