Okay, I've gotta chime in here and bring something bigly up. If you have the "fusion drive" version - it's actually 2 drives. The this guide only covers the replacement of the top, easy to get to one. You'll have to go deeper into the rabbit hole if you want to replace the other, or spare, or spinner drive.
It wasn't until I had to change out my IR sensor that I realized I had a drive just $@$*!&%& there... I used the "IR Sensor Guide" to get the job done:
Mac mini Late 2012 IRセンサーの交換
I guess you can use the "Dual HD Kit Guide" if you want to "upgrade" or "install" the "kit."
Mac Mini Late 2012のデュアルハードドライブキットのインストール
