Removable of the existing HDD to install the SSD is pretty easy. Just follow the teardown guide on iFixIt. I've never actually done an upgrade, I only like taking apart my computer for no good reasons, so I don't know if the Runcore drive comes with the rubber bumpers and foam padding or not. If said things do not come with the drive, you'll need the remove the existing ones from the current drive.
As far as removal goes, I removed the flip-down hatch in order to not have to disconnect a bunch of delicate ribbon cables. To remove the hatch, you'd need to first remove the speaker (which is 2 screws)you will feel some resistance when lift the speakers up, and then remove the 4 screws to the latch which moves easily once the screws are removed. Finally the HDD is helding by 4 screws2 next to the battery and the other 2 are under a wire which can be moved easily if you first remove the screw from the attachment next to the hinge.
I think my description is might be confusing, so I'd take a look at the iFixIt teardown, match what I described, then go over it on your own computer (by removing the bottom cover) in order to see each step and how you want to proceed. Again you can remove the HDD without removing the speakers and hatch, but you'd have to disconnect some delicate ribbon cables. I'd go over these before you decide whether or not to upgrade.
Please keep the screws apart because the ones for each hole is not the same. I'd use an egg crate if possible.