Many of Asus f555la reviewers do mention that one of the con of this laptop is the hard drive, which is a 500 GB 5400 RPM HDD, check out this post, if you want to get the similar model with a 1tb HDD.
Another affordable model, which is the k501lx, has even SSD already installed.
There was also someone who commented in the Asus x550za review post, asking if he could missed something in order to open the laptop panel.
So I recently found a good video tutorial on how to do this, and here are a few things you will need.
Things to prepare before replacing HDD
1. Small screw drivers
2. Your new Hard Drive, can be a SSD.
3. A soft cloth, in case you want to invert your laptop upside down.