The prices Apple charges for upgrades (hard drives, SSDs, as well as memory) are unconscionable. But with their tiny parts and tolerances crammed into a tiny space, it is daunting to tackle on your own. Their teardown showed me it was possible, and their kit makes it look professionally installed.
Chunks of my hair would have been missing without this kit and guide. Huge thanks to iFixit for putting in a lot of time and effort to make this as straightforward as possible.
Their kit + guide + 1 hour = Much happier mini owner. Now I have a speedy and spacious 1TB for my HT.
There was only one part that was difficult for me (and probably only me). In one of the last steps before putting the shields and fan back on, I had trouble getting the top hard drive in. Upon peering into the space, I realized there were two grommets on the inner part of the case that accept the pegs from the upper hard drive, and I actually managed to push one out its space! (Don't do this). It took a while to get it back in, since you have to use a tool to move the rubber grommet around in the recesses of the case.
The moral is: if the upper hard drive doesn't fall back into its slot when putting it back in, gently move it back and forth to allow the pegs to find the grommets. Don't force it.