Can i upgrade from 10.9.5 to 10.12 on my mac mini overwriting 10.9.5 or do i have to have to erase the disk and start again?

No problem, you're welcome!

Actually your best bet for compatibility, reliability, guaranty and support is to use OWC for the SSD, the Memory and drive Enclosure. https://eshop.macsales.com/Service/

What year model is your Mac Mini?

If you have a 2012 or later Mac Mini, then you can download, install and run Catalina.

see > How to upgrade to macOS Catalina - Apple Support

If you have a 2010 or 2011 Mac Mini, then you can download, install and run High Sierra.

see > How to upgrade to macOS High Sierra - Apple Support

If you have a 2009 Mac Mini, then you can download, install and run El Capitan.

see > How to upgrade to OS X El Capitan - Apple Support

No you do not need to erase your disk. All you need to do is download and install the new macOS. Plus before upgrading to a new macOS, it is always a good idea to have a Time Machine or Clone backup on an external drive.

RogueMale wrote:

Late 2012.

A late 2012 can run Catalina, but it will be really slow if it still has the stock spinning hard drive and only 4 GB of RAM.

i am nervous that this update will nerf my mac and slow it down! it is functioning really well fast and flawlessly, although some software that i wish to use wont run anymore,

Yea I totally understand and that is why it is important to have a good backup on a reliable external drive or two.

At this point if the Mac Mini is still stock and you run older software, then you should only take it up to High Sierra.

Is it possible to down-date the OSX back to 10.9.5 if the Catalina update "ruins" my perfectly functioning mac?

It is, but it's a real pain in the *** that requires erasing the drive, reinstalling the old macOS and then migrating your data from a worthy backup.

thanks

You're welcome!

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FWIW I have 2 2012 Mac Mini's that I have upgraded to SSD's and 16GB of RAM. They run the later macOS Mojave and Catalina with no problem whatsoever.

If you are considering upgrades for your 2012 Mac Mini so that you can go to Mojave or Catalina. Please check out the Memory and SSD upgrades at >https://eshop.macsales.com/shop/memory/Apple/Mac_mini/DDR3_1600