Os upgradation

The upgrade to High Sierra 10.13.6 is available from a 'download

installer' page through an Apple site; among other older systems.

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

macOS High Sierra 10.13 hardware requirements

Download installers get only the installer, then it brings

the parts of the system to your mac and installs it there.

"Safari downloads the following older installers as a disk image

named InstallOS.dmg or InstallMacOSX.dmg. Open the disk

image, then open the .pkg installer inside the disk image..

It installs an app named Install [Version Name]. Open that app from

your Applications folder to begin installing the operating system."

The following 'active link' goes and gets this High Sierra installer, to start the process.

macOS High Sierra can upgrade Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion

Good luck & happy computing! ☃️🌿❄️🌻

It looks like High Sierra would be the last update for your 2011 Mac:

https://support.apple.com/en-ca/HT211683

My experience(s) show the High Sierra hasn't slowed my Late

2012 Mac mini 2.3GHz (server) quad-core + dual 1-TB HDDs.

First item after I bought Mac from Apple Clearance (October

2014, new.) Memory upgrade (from 4GB to) 16GB from OWC.

Each of four systems are on 500GB partitions; and I have a few

USB drives as backups for Time Machines. (+ three other Macs.)

Given this original server with two stock 1-TB rotational HDDs

will be slower than expected once I use Catalina for awhile, as

these are 5400-RPM drives ~ should have 7200+ RPM, or SSDs.

Mojave is resident in HDD2 partition 2.

The upgrade from El Capitan to High Sierra near a month ago

was due to few or no choices with secure web browsers. Also to

continue to use webmail, put added load; older browser froze!

Safari? Kinda works; sites may have 'certificate expiry' issues.

Firefox 91.4.1 ESR for High Sierra is available from Mozilla:

"Get Firefox for your enterprise with ESR and Rapid Release"

https://download.mozilla.org/?product=firefox-esr-latest-ssl&os=osx&lang=en-US

Choose Firefox ESR from drop-down menu

Firefox Extended Support Release (ESR)

I've newer Firefox ESR browser. Tried older Mozilla 'client email.'

With several things off-tracks at once, its going OK now anyway.

My preference had been El Capitan; Firefox had no update. Was

able to get new replacement Certificates, from "Chain of Trust -

Let's Encrypt" from reliable source in October 2021. [An upgrade

over El Capitan, left the original Mavericks 10.9.5 Server, still.]

Wow.. looks like time is near 2:30AM here.

Have to go & consider sleep too.

Take care & safe travels! ☺︎