My point is if you damage the mini during install, then you're not covered and they'll know you damaged it.
Those little wires and small onboard headers are a very risky operation, well worth an Apple Store installing it at least.
Changing a hard drive does not void warranty. That's a long-established principle.
That's a completely different question to that of whether the OP is competent to do the work, and from what little they've posted it's impossible to tell.
Warranties cover defects in manufacturing, not user abuse. If you break something, it's not covered by warranty. So with older minis with field replaceable ram, for example, if you break the socket by being ham fisted, it's not covered under warranty.
I changed the hard drives in both my minis the other day. It wasn't particularly difficult. But then, I say that as an electronics engineer with 20 years experience in working in surface mount electronics.
In France, my local Apple Store don't do upgrade, directed me to Apple Authorized service providers.
The Apple Store in the USA do upgrades on Mac Mini ?
How much does it typically cost to add a 2nd drive ?
Apple stores will only install the exact factory parts. When my iMac broke the Apple store wanted $475.00 to replace both the 1TB Seagate HDD and the optical drive.
Just added a 240GB M500 SSD to my Late '12 Mac Mini.
Bought the Dual Drive Kit from OWC and was good to go. Super simple process.
I found the hardest part was getting the Power Supply Cable off the logic board. Other than that, really took my time and just went easy.
Used internet recovery and installed Mavericks on it. Will keep 10.9 on it for now.