At work we had a mac mini 2014 that needed an ssd harddrive upgrade. This post has a short video and some info on the process documenting how to remove the harddrive from a 2014 mac mini and install an ssd.
You will need a few tools:
The first thing you will to do is to prep your install media – just go to the mac appstore and find the latest version of osx. click download. Once the download has finished, the installer will auto-start – you should quit the installer – you don’t want to reinstall on your mac!.
I used the createinstallmedia command to create a bootable version of high sierra, using instructions from apple support
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume
once the installmedia has been created (this took about 30mins you can start the teardown process. I followed the instructions from ifixit which were very good (though it would have been easy enough to figure out without them, there are only a handfull of screws and most things click into place).
I recoreded the process of how to remove the harddrive from a 2014 mac mini and install an ssd on my phone, and made a short video. if you want to watch (its double speed, so only lasts 3 mins. the whole process took about 15 mins to actually replace the drive)
Once the drive was in all that was required was a standard install from the usb boot media created earlier.