Can I upgrade my iMac Mid 2011 running El Capitan 10.11.6 to High Sierra safely?

You should be able to safely. However, if you have any any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac you should uninstall them according to the developers instruction before upgrading to High Sierra.

Yes. Any Mac running 10.11 can run 10.13. Our 2011 iMac 21,5" is quite happy with 10.13.

Are you asking because your saw the advisory shown in the Apple article linked above your name? The wording of the advisory and the article are woefully unclear, but all that does is download a small patch for iTunes so it can manages newer iOS and iPadOS versions. It does not change your mac OS version.

I would recommend making a bootable backup of your El Capitan startup disk, and setting the backup aside – in case something went wrong with the upgrade. (You make the backup in hopes that you won't need it. It is when you fail to make the backup that Murphy's Law strikes, and you find out that you do need it!)

Once you have High Sierra up and running, you may want to install the Firefox 115 ESR Web browser. It is newer than the versions of Safari in El Capitan and High Sierra, so might help with browsing some Web sites.