Need to upgrade to High Sierra. Asking for help.

I've been running 10.13.6 High Sierra on my mid 2011 for years.

Using your safari browser only go to this link:

https://apps.apple.com/us/app/macos-high-sierra/id1246284741?mt=12

That should open the app store on the High Sierra download page.

Sorry, but you won't be able to upgrade your Mid 2011 iMac to macOS High Sierra as it is not compatible with that OS version. You would need a 2012 or newer iMac for that.

Ref: https://support.apple.com/kb/SP765?locale=en_US

High Sierra will run on a 2011 iMac. Our 2011 iMac is quite happy with it. It is 10.14 Mojave that requires a 2012 or higher Mac.

High Sierra System Requirements

You can get High Sierra from a link in this Apple article:

How to get old versions of macOS - Apple Support

Opps, sorry about that. I used the info farther down on that article by mistake. The other responders, thankfully, got you the right answer. Not enough coffee this morning ... ugh!