OS Sierra
I have an iMac 21.5" Late 2015, what OS can I update to
iMac 21.5″, macOS 10.12
Hi there!
You can upgrade to macOS Monterey.
How to download macOS - Apple Support
Before installing, it’s a good idea to back up your Mac. While installing, avoid putting your Mac to sleep or closing its lid. Your Mac will restart, and it might show a progress bar or blank screen several times.
Jack