A detailed video with instructions on how to remove the screen of your 21.5” iMac in order to upgrade the internal hard disk drive (HDD) to a SATA connected Solid State Drive (SSD)

Goal: To increase the iMac’s speed and longevity, while decreasing the time it takes to boot up and load your files.

We used a brand new Samsung Evo 860 V-NAND MLC 500GB Solid State Drive with a SATA 6Gb/s Connector. (Not the M.2 Model)

Note: Some systems support a smaller PCIe-based “blade” SSD, as well. Specifically, if the 21.5-Inch "Late 2012" or "Early 2013" iMac models were configured with an SSD or a "Fusion Drive" at the time of purchase, the SSD module is connected via a dedicated PCIe connector and another m.2 “adapted” SSD can be installed in its place. Unfortunately, if the system only was configured with a hard drive, the needed connector is not present on the motherboard and there is not a way to add a PCIe-based "blade" SSD afterwards.
However, the 21.5-Inch "Late 2013" iMac models do have the connector for a PCIe-based "blade" but subsequent 21.5-Inch models -- like the entry-level iMac "Core i5" 1.4 21.5-Inch (Mid-2014) and the 21.5-Inch "Late 2015" models -- do not have the connector for a PCIe-based "blade" SSD if one is not installed at the time of initial system purchase.
By contrast, the 27-Inch "Late 2012" and "Late 2013" as well as the "Late 2014" and "Mid-2015" Retina/5K iMac models all have this connector on the motherboard even if the system only is configured with a hard drive at the time of purchase, so it is possible to add a PCIe-based "adapted blade" SSD to these models after the initial system purchase.

Required parts:

-SSD of your choice (SATA Connector, 6.0Gb/s V-NAND MLC or TLC)
https://www.newegg.com/samsung-860-evo-series-500gb/p/N82E16820147674

-The complete set of replacement adhesive strips are specifically cut to fit 21.5" iMacs from Late 2012 through 2019.
https://www.ifixit.com/Store/Mac/iMac-Intel-21-5-Inch-2012-2019-Adhesive-Strips/IF173-005?o=1#productDescription

-Painters Tape for holding screen to frame

-T10 Torx Screwdriver
https://www.ifixit.com/Store/Tools/TR10-Torx-Security-Screwdriver/IF145-080

-iMac Opening Tool
https://www.ifixit.com/Store/Tools/iMac-Opening-Wheel/IF145-219

-iMac Service Wedge
https://www.ifixit.com/Store/Tools/iMac-Service-Wedge/IF145-224

-Spudger
https://www.ifixit.com/Store/Tools/Spudger/IF145-002

-Medium / Small Screwdriver, “+” cross (Phillips)

-Plastic Card or Guitar Pic

-OPTIONAL
SATA to USB 3.0 Adapter cable (for data transfer)

Here we perform an upgrade on the iMac by removing the gaskets around the screen, cleaning out any fans in the system, removing the existing hard drive, and then replacing it with a solid-state drive. We format the drive and then install macOS. We replace the gaskets and seal up the iMac after verifying the installation. The last optional step would be to clone your existing installation over or use the migration assistant to transfer your files over to the new SSD with a SATA to USB adapter.