Opening the Retina iMac's can be tricky and you need the right tools. And you're going to need a few things to open the case and to install the SSD correctly.

Here's the current iFixit tear down that shows you what you're getting into. iMac Intel 27" Retina 5K Display Teardown

And from our friends at OWC, they have a great video on taking the HDD complete out and replacing it with an SSD, but this guide will show how to simply leave the HDD and install the SSD. https://vimeo.com/139364064

Make sure to grab the essential tools like: Pro Tech Toolkit and Jimmy for opening up the iMac, although I used a medical scalpel, I found that the "guitar pick" was too weak and too thick. The "Jimmy" would be a great substitute for a scalpel.

You will also need the OWC In-Line digital thermal sensor, so that the iMac temperature control stays under control. Otherwise you fans will spin at full speed. This OWC kit also includes the needed tape to close you iMac back up. https://eshop.macsales.com/item/Other%20...