So, I decided to replace the hard disk with an SSD. I searched Google for a disassembly instruction. Unfortunately, I could not find any. After struggling with the laptop for a few hours, I finally figured out how to do that. I would like to share my experience to help others who would like to do the same thing.
First, you need to turn the laptop upside down. There are 10+ small screws. You need to unscrew all of them. There is a screw with a DVD logo beside it. If you remove it and the one near the DVD drive placeholder, you can remove the placeholder. I think you can install a secondary hard disk, if you wish.
If you remove the DVD drive placeholder, there will be a large enough gap to put your fingers into. Put your fingers there and pull the the upper plate hard. The upper plate is stick to the bottom plate by several plastic hooks around the edge. You will not break it, just pull hard. But do not try to pull the upper plate completely, because there are 3 flex cables from it connected to the main board. You just have to pull enough to unhook those hooks.
The 3 flex cables have common shapes with other flex cables you would find in other devices. The left-most one is the power connector, the middle one is the trackpad, and the right-most one is the keyboard. For the power connector, lift up the black cover and it can be easily pulled. For other two connectors, pull the small pins on both sides, and the cables will be removed.
Now that all the cables have been removed, you can finally take the upper plate completely. As you see below, the hard disk is located on the left.
There is a screw under the cable to the fan. You need to unscrew it in order to remove the disk.
Once you have replaced the disk with an SSD, you can do assemble the laptop again connecting the parts in reverse order.