how do i update safari on my iMac

Which exact Mac model do you have, and what version of macOS is it currently running?

As dialabrain has already mentioned, you may have to upgrade to a newer version of macOS in order to upgrade the version of Safari.

If the issue you are having with your current Safari browser version not being able to access certain secure sites, you basically have two choices:

1. Use another web browser, that still provides the proper security, with the version of macOS running on your Mac, OR
2. Upgrade macOS. If not possible, then upgrade upgrade your Mac itself.

The latest version of Safari for macOS Catalina is 15.6.1. 16.6.1 is part of macOS Big Sur, the next version upgrade.

Ref: Update to the latest version of Safari - Apple Support

Again, if you need to run v16, you will need to upgrade to, at least, macOS Big Sur. If your Mac does not support running Big Sur, again, you can try a different web browser, like Brave or FireFox.

FWIW, v17 is available for macOS Monterey thru Sonoma.

The only way is to update macOS.