Once you have the back off of the machine, ignore everything except for the area in the picture. Yours may differ slightly, but this is your SSD.

I recommend gloves for this (ignore my un-gloved hand!). Nitrile or Latex is fine, but make sure they are NOT powdered! Also, be careful not to catch and rip them on anything inside.

You will have one more screw to remove (Orange circle) before you can remove the drive. This screw will be here no matter what type of SSD is already installed.

Once you have removed the screw, you can remove the SSD. Do this by first, gently pulling UP by where the screw was; and then pulling AWAY as shown in the picture with green arrows. Be gentle and do not force it.