I'm on OS X El Capitan, Should I upgrade to a newer version?

The mac model and age will determine which OS's you can upgrade to.

If your mac is ten years old then the highest OS you can probably upgrade to is macOS High Sierra.

macOS Mojave and macOS Catalina require a mac to be from 2012 or newer.

The newest macOS Big Sur requires even newer macs.

To find out your macs year click on the top left of your screen

and click on About This Mac.

Open your Safari browser, do not use Firefox and

click on this link for a guide on upgrading your mac,

How to get old versions of macOS – Apple Support

Go to Check Compatibility click on the OS you would like

to upgrade to and see if your mac qualifies.

Then scroll down to Download OS and click on the OS

you want.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS High Sierra.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

Mac's running El Capitan are supposed to be able to run Catalina, which means you have a few higher level of OS's that should be able to run on your Mac Mini. Of hand I think there are Sierra, High Sierra, Mojave, Catalina. However, my experience upgrading to later levels of OS's on old hardware is not all that great. Plus, you run the risk of not being able to go back to El Capitan if you want to. Ok, it can be done but you really have to search to find out how to do it as Apple is more of a one way upgrade company. Might depend on how tech savvy you are and your level of risk acceptance.

El Capitan, while not being very "iOS"-like, is a very stable OS. The biggest issue with it now is that day by day its becoming incompatible with upgraded apps from third party suppliers (Microsoft, Adobe, etc) as they release newer apps made for more recent OS's. Apple only supplies security patches for El Capitan and nothing else and if you contact Apple Support, they really frown upon helping people running El Capitan yet will do it as much as they can. Safari under El Capitan is becoming incompatible with some websites and will exhibit strange behavior at times.

I run El Capitan on my 2011 iMac. I upgraded to High Sierra and it destroyed my internal graphics card and due to this I was able to go back to El Capitan after replacing my graphics card. I will simply stay with El Capitan until I buy a new Mac. This all being said, I give Apple a lot of credit for allowing a 10yr old computer to run very well on aging software, I mean in the high tech world, everything is pretty much obsolete in much less time.

Summary: Yes, you can upgrade to a later OS but may not easily be able to go back to El Capitan if you want or need to. My experience is if you do upgrade you may well find a few issues to solve so you will need to systematically go through all the apps you use and test them, including all your email accounts.

Hope this answered your question :-)

Here is a link where you can find older OS versions should you decide to upgrade and then go back to El Capitan

How to get old versions of macOS - Apple Support