Easier than you think because the hard drive is located just under the perforated stainless plate. OWC has excellent videos on the subject but they can be misleading because they "add" a second drive to the Mini and that takes an involved surgery, necessitating the removal of the logic board. Just to replace the existing hard drive with an SSD is much simpler on the other hand.
Remove the black bottom plate, no need to remove the memory modules, remove the fan (only three T5 torx screws), gently detach the connector by pulling on its wires. Remove the perforated stainless which holds the WiFi antenna by unscrewing the two outer screws then removing the smaller flush screws on the same plate holding the HDD in place (use Torx T8). Flip the perforated plate toward left without removing the WiFi wires. Remove the fan duct by removing its T5 screw on the bottom (the crescent shaped black plastic to the left) It takes a bit wiggling to get it out of there.
There is a flush square connector connecting the HDD to the main board. Gently pry it out with your nails from underneath or use a plastic prying tool. The hard drive is ready to come out now. It is a tight fit but if you pull it towards the center while sliding out and wiggle at the same time, it will come out with minimal problem. You may leave the black protecting film on the HDD as SSD's have no exposed circuits underneath, you won't need it. Remove the black screws on the side of the HDD and put them on the SSD. Remove the cable connector from the HDD and put it on the SSD.
Reverse the procedure by sliding the drive in. Re connect the drive to the main board with a gentle snap to make sure it is in place. Put the fun duct back by wiggling and sliding in place. Place the perforated plate and fasten the screws in reverse order. Do not over tighten any of the screws on the plate. The inner flush ones with rubber grommets on the plate holds the drive in place and the outer screws are for holding the black round cover in place. First re connect the fan to the board by a gentle push until you hear a click then tighten the three screws of the fan back.
I timed myself and it took me 3 minutes to get the drive out and 7 minutes to complete the job. Everything is a tight fit and needs gentle wiggling to put things in place. Do not use excessive force as it's not needed.