Started 7th January 2023
I have had a Mac Mini (late 2012) edition from new. As time has gone by, it has got slower to the point where it is irritating to use. From comments on the web and observing its behaviour I came to the conclusion the hard drive is the problem. I decided to replace the drive with an SSD.
How to do this is described in this YouTube video:
https://www.youtube.com/watch?v=BP3oPq1I-iQ&t=168s
Take out the drive, put in a blank SSD, and surprising to me, it offers to reinstall its original OS on the new drive - I got OS X Mountain Lion. The problem that then arises is that Safari complains it can't make a secure connection to various web sites, in particular Apple's so getting further upgrades is not possible.
I found this web page useful:
https://discussions.apple.com/thread/253250487
Various possibilities are described, I downloaded Sierra (which does not need https) and from there I could upgrade to Catalina.
The Mac Mini now runs a lot faster, using it is as pleasant as it should be. I am not using it for anything ambitious.
The sad bit is that having got it working I found software support was discontinued in November 2022, so the machine is not safe to use on the internet.