It is very easy to replace the internal hard drive in a Mac mini, mine is the late 2012 model. I put my 1TB ssd in a case using a usb cable to hook up to the Mac. I used carbon copy to duplicate all the data from the internal hard drive to the ssd. After that was done, you reboot with the option key held down and tell the system you want to boot from the ssd. I did this for weeks so I can verify that everything worked before swapping the ssd/hard disk.
I then followed online instructions I think from OWC to remove the hard disk and put the ssd in the mini. Probably took me 15-20 minutes. Kept the hard disk around for a while to make sure everything worked.
I didn’t use any special tools to take apart the Mac mini. I had small screwdrivers that allowed me to remove all the screws. Been running for over a year without any issues.