update an OS X EI Capitan version 10.11.6

El Capitan is so old that it has a problem with expired root certificates, and a Safari web browser that has not received updates in many years.

Additionally, there are root certificates that expired last week, which will impact the usability of Firefox 78 ESR - which is the latest version of Firefox that can be used on El Capitan. Similar issues may occur with other Chromium-based browsers shortly, unless they are specifically made to work with older macOS. Your options are limited...

So my recommendations:

• get any outstanding updates for your El Capitan install by running software update from the command line:

sudo softwareupdate -ia

• There is an auto-downloader for Chromium Legacy Browser. I'm not the biggest fan because it was deseign for OS X 10.7-10.9 and can be a bit flaky of El Capitan...but it will do a bunch of background fixes in addition to downloading Chromium Legacy. Most importantly, it will update your ISRG X1 Root Certificate. You can direct download it here:

https://jonathanalland.com/downloads/chromium-legacy-downloader.dmg

Even with all that, El Capitan is rapidly approaching the terminal end-of-life stage of its predecessors, where it is basically just a hobby toy now and you can't do anything substantial with it anymore.

El Capitan has already reached its end of life. The only 3 systems that will be getting system and security updates are Ventura, Sonoma and Sequoia. Once the next system is released Ventura will be unsupported.

g_wolfman's suggestion of using Firefox or a Chrome based browser like Brave is the way to go.