my mac last update macOS Sierra 10.12.6

Is your question how to update your iMac to the latest OS (which is High Sierra 10.13.6)? If so, follow the steps below:

1) Backup your iMac with Time Machine before you begin (see this link on how: https://support.apple.com/en-us/HT201250)

2) Click here to go to the older macOS download page, scroll down and click the High Sierra link.

3) The link should redirect you to an App Store download for High Sierra, and this will download the installer.

4) Open the installer and follow its instructions to install macOS.

Let me know if this is what you wanted to know.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

macOS High Sierra 10.13 hardware requirements Pretty sure High Sierra macOS 10.13 it the last Version it can run.

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation