Why do you wish to upgrade the RAM in your mini?
Are you having some performance issues with the computer?
If so, you should know that very often it isn't RAM that is the problem. Instead, it is something other, such as software conflict or hardware compromise.
I recommend that you run an EtreCheck evaluation of your Mac. EtreCheck is a diagnostic app that gathers important info about your hardware and software and reports it back to you. It is very highly regarded and meant to be used with Apple Support Communities to help others help you troubleshoot and resolve your Mac problems. It will gather no personally identifiable info.
Please navigate to EtreCheck.com and download the free version of EtreCheck. When you run the app it may request full disk access, please grant that so it can give a thorough report. Once you’ve run the app and created your report please post it with your reply to this message. You must upload the full report. To see how, please click > How to use Add Text when posting… EtreCheck Report.
We can use the EtreCheck report to look for the more obvious things that may be causing your problem and advise how to correct them.