New system update

Use the 🍎 > About this Mac, and post the iMac model year here, and we can post back what your upgrades options might be, if any.

Or you can research those options yourself: How to get old versions of macOS - Apple Support

macOS Monterey is current, and an iMac from 2015 and newer can upgrade to that.

macOS High Sierra is unsupported and isn’t getting patches, and lacks various security features added more recently including the read-only system volume and anti-malware updates including XProtect Remediator.

What year model is your iMac?

see > Identify your iMac model - Apple Support

You will need to have

a Late 2012 iMac to upgrade to Catalina 10.15

a Mid or Late 2014 iMac to upgrade to Big Sur 11

and a 2015 or newer iMac to upgrade to Monterey 12

For more on macOS upgrades and requirements,

see > How to get old versions of macOS - Apple Support

Brady1287 wrote:

Hi, thanks for replying to my question. Yes, its an old but reliable one - 21.5 inch mid 2011. Not sure what to do to keep the hackers away. I've never had any issues to date.

21.5" iMac Mid 2011 tops out at High Sierra.

You're four and very soon five macOS versions back.

There have been security fixes since then, yes.

Do keep scheduled backups too, as the hard disks in those iMacs are getting old and can fail. A failing hard disk drive from that era usually shows first as decreased performance and as beachballs, then hangs and crashes, and then... well... have backups.

If your Mac is not compatible the only thing you can do is acquire a Mac that is.