Generally, the CPU in a notebook is soldered to the board and can't be replaced. The Asus K501LX-DM028D is probably not an exception to this.
Adding an SSD usually requires removing the hard drive, unless there are two slots for storage drives. The Asus K501LX-DM028D does not have two slots as far as I can tell from a Google search for its specifications. You would need to get windows onto the SSD if the computer didn't ship with the SSD installed and that can be a bit of a hassle with laptops. You'll either need to get your own copy of Windows and install it or copy the hard drive's data to the SSD with an SSD migration or partition editing program. The problem is that you need both the SSD and the hard drive connected to a computer at the same time to do this and the laptop has no eSATA ports, so you'd either need to get a SATA to USB adapter or plug both the laptop hard drive and the new SSD into a desktop computer and run the copy on that computer.
Some SSDs ship with the adapter, some do not. If not, then it's not very expensive and would be the easiest way to do this because the other option still requires buying SATA cables and opening up a desktop computer to plug in the drives.
Getting an SSD would probably be the only upgrade you can do since you've already got the memory maxed out. The CPU probably isn't replaceable and the graphics probably isn't either.