My 1 TB drive was getting full with videos, pictures, and music. To buy me another five years of life on my iMac, I decided to double storage to 2 TB using the Seagate drive recommended online.
It's easier than it looks -- I taped each screw to a paper along with noting the step, however it probably wasn't necessary as there are only two sized Torx screws, and not that many once you get off the first 8 that hold the video display. I also stored the glass and video display between layers of newspaper as someone else mentioned. The hardest part was disabling and reattaching the tiny cables before taking out the actual hard drive, and with a little patience and a good flashlight, they came off without a hitch. I suggest using a box to keep the video display lifted during the middle process steps.
I wasn't clear on what to do after the HD was hooked up, but Apple makes it easy. Hold down CMD and "R" keys and wait for the internet reinstall procedure. Also, make sure you partition your hard drive in Disk Utilities before proceeding to loading up the Time Machine backup. I suggest hooking up directly to the Time Machine with cable instead of waiting for the sluggish wi-fi backup. Finally, I needed to buy the HD fan control utility, which was overpriced at $35, but was an instant fix to the horrible noise of the fan running at 6000 rpm.