IF it is the MacBook, and not a MacBook Pro (some people call them both a MacBook) then you have to remove the battery, undo 3 screws, and then slide the hard drive out of the machine. Get yourself a T6 screwdriver, and remove the old drive from the drive sled, then put the new drive in. Reverse the process.
My understanding of the situation is that anything that can be removed is technically user replaceable. So, in the case of the MacBook, the processor is not user replaceable, but the wireless card, the hard drive, the optical drive, the RAM are all technically user replaceable.
Apple is under no obligation to support the new hard drive that you put in, which is reasonable, so should a problem arise, be prepared to put the old drive back in.